Simplify mock clock in some unit tests

[kpschoedel]

Problem

PR #12895 included consolidating several unit tests' nearly-identical
mock System::Clock implementations into one, but missed a couple.

Change overview

Use System::Clock::Internal::MockClock in TestDnssdCache
and TestPeerConnections.

Testing

Yes indeed.

[github-actions]

PR #13162: Size comparison from 569e7cc to ee486c8

Full report (31 builds for efr32, esp32, k32w, linux, mbed, nrfconnect, p6, qpg, telink)

platform	target	config	section	569e7cc	ee486c8	change	% change
efr32	lighting-app	BRD4161A	(read only)	830408	830408	0	0.0
			(read/write)	127360	127360	0	0.0
			.bss	125480	125480	0	0.0
			.data	1876	1876	0	0.0
			.text	830400	830400	0	0.0
		BRD4161A+rpc	(read only)	818052	818052	0	0.0
			(read/write)	144024	144024	0	0.0
			.bss	142048	142048	0	0.0
			.data	1976	1976	0	0.0
			.text	818044	818044	0	0.0
	window-app	BRD4161A	(read only)	803784	803784	0	0.0
			(read/write)	126296	126296	0	0.0
			.bss	124464	124464	0	0.0
			.data	1832	1832	0	0.0
			.text	803776	803776	0	0.0
esp32	all-clusters-app	c3devkit	(read only)	881716	881716	0	0.0
			(read/write)	1312466	1312466	0	0.0
			.dram0.bss	69296	69296	0	0.0
			.dram0.data	14220	14220	0	0.0
			.flash.rodata	175896	175896	0	0.0
			.flash.text	881716	881716	0	0.0
			.iram0.text	62254	62254	0	0.0
		m5stack	(read only)	942151	942151	0	0.0
			(read/write)	441576	441576	0	0.0
			.dram0.bss	73792	73792	0	0.0
			.dram0.data	34056	34056	0	0.0
			.flash.rodata	202720	202720	0	0.0
			.flash.text	936767	936767	0	0.0
			.iram0.text	122671	122671	0	0.0
k32w	light	k32w061+release	(read/write)	648740	648740	0	0.0
			.bss	76488	76488	0	0.0
			.data	1904	1904	0	0.0
			.text	564548	564548	0	0.0
	lock	k32w061+release	(read/write)	633456	633456	0	0.0
			.bss	76208	76208	0	0.0
			.data	1860	1860	0	0.0
			.text	549588	549588	0	0.0
linux	chip-tool-ipv6only	arm64	(read only)	7005564	7005564	0	0.0
			(read/write)	326225	326225	0	0.0
			.bss	54593	54593	0	0.0
			.data	1096	1096	0	0.0
			.data.rel.ro	210144	210144	0	0.0
			.dynamic	560	560	0	0.0
			.got	56696	56696	0	0.0
			.init	24	24	0	0.0
			.init_array	168	168	0	0.0
			.rodata	384228	384228	0	0.0
			.text	5928084	5928084	0	0.0
	thermostat-no-ble	arm64	(read only)	2002756	2002756	0	0.0
			(read/write)	144065	144065	0	0.0
			.bss	64353	64353	0	0.0
			.data	880	880	0	0.0
			.data.rel.ro	72088	72088	0	0.0
			.dynamic	560	560	0	0.0
			.got	3840	3840	0	0.0
			.init	24	24	0	0.0
			.init_array	296	296	0	0.0
			.rodata	128356	128356	0	0.0
			.text	1662736	1662736	0	0.0
mbed	all-clusters-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2338232	2338232	0	0.0
			.bss	188580	188580	0	0.0
			.data	5272	5272	0	0.0
			.text	1300808	1300808	0	0.0
	lighting-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2330568	2330568	0	0.0
			.bss	180920	180920	0	0.0
			.data	5552	5552	0	0.0
			.text	1293168	1293168	0	0.0
	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2303664	2303664	0	0.0
			.bss	179968	179968	0	0.0
			.data	5544	5544	0	0.0
			.text	1266264	1266264	0	0.0
	pigweed-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	1140008	1140008	0	0.0
			.bss	11756	11756	0	0.0
			.data	4368	4368	0	0.0
			.text	103392	103392	0	0.0
	shell	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2053688	2053688	0	0.0
			.bss	156972	156972	0	0.0
			.data	4864	4864	0	0.0
			.text	1016288	1016288	0	0.0
nrfconnect	lighting-app	nrf52840dk_nrf52840	(read/write)	935999	935999	0	0.0
			bss	118408	118408	0	0.0
			rodata	108124	108124	0	0.0
			text	631884	631884	0	0.0
		nrf52840dk_nrf52840+rpc	(read/write)	922351	922351	0	0.0
			bss	115452	115452	0	0.0
			rodata	101540	101540	0	0.0
			text	627196	627196	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	859714	859714	0	0.0
			bss	116692	116692	0	0.0
			rodata	103048	103048	0	0.0
			text	559324	559324	0	0.0
	lock-app	nrf52840dk_nrf52840	(read/write)	908095	908095	0	0.0
			bss	117596	117596	0	0.0
			rodata	103428	103428	0	0.0
			text	609708	609708	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	832058	832058	0	0.0
			bss	115908	115908	0	0.0
			rodata	98392	98392	0	0.0
			text	537192	537192	0	0.0
	pigweed-app	nrf52840dk_nrf52840	(read/write)	542351	542351	0	0.0
			bss	52588	52588	0	0.0
			rodata	50668	50668	0	0.0
			text	376892	376892	0	0.0
	pump-app	nrf52840dk_nrf52840	(read/write)	909391	909391	0	0.0
			bss	117356	117356	0	0.0
			rodata	103644	103644	0	0.0
			text	610948	610948	0	0.0
	pump-controller-app	nrf52840dk_nrf52840	(read/write)	906207	906207	0	0.0
			bss	117384	117384	0	0.0
			rodata	102900	102900	0	0.0
			text	608456	608456	0	0.0
	shell	nrf52840dk_nrf52840	(read/write)	796079	796079	0	0.0
			bss	109464	109464	0	0.0
			rodata	78096	78096	0	0.0
			text	532048	532048	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	708710	708710	0	0.0
			bss	107352	107352	0	0.0
			rodata	72396	72396	0	0.0
			text	449544	449544	0	0.0
p6	all-clusters-app	default	(read/write)	2389936	2389936	0	0.0
			.bss	116780	116780	0	0.0
			.data	2552	2552	0	0.0
			.text	1348200	1348200	0	0.0
	light-app	default	(read/write)	2324944	2324944	0	0.0
			.bss	106160	106160	0	0.0
			.data	2384	2384	0	0.0
			.text	1283208	1283208	0	0.0
	lock-app	default	(read/write)	2297152	2297152	0	0.0
			.bss	105040	105040	0	0.0
			.data	2336	2336	0	0.0
			.text	1255416	1255416	0	0.0
qpg	lighting-app	qpg6105+debug	(read only)	532240	532240	0	0.0
			(read/write)	146936	146936	0	0.0
			.bss	86824	86824	0	0.0
			.data	1004	1004	0	0.0
			.text	526920	526920	0	0.0
	lock-app	qpg6105+debug	(read only)	503904	503904	0	0.0
			(read/write)	146940	146940	0	0.0
			.bss	85960	85960	0	0.0
			.data	952	952	0	0.0
			.text	498584	498584	0	0.0
	persistent-storage-app	qpg6105+debug	(read only)	106448	106448	0	0.0
			(read/write)	146938	146938	0	0.0
			.bss	36146	36146	0	0.0
			.data	288	288	0	0.0
			.text	101128	101128	0	0.0
telink	lighting-app	tlsr9518adk80d	(read/write)	832258	832258	0	0.0
			bss	87064	87064	0	0.0
			noinit	37160	37160	0	0.0
			text	580384	580384	0	0.0