tests: Fix flaky "DHCP RELAY". #4685

Job	Run time
Prepare container	3m 34s
linux rpm fedora	6m 47s
osx clang --disable-ssl	1m 58s
linux gcc --disable-ssl	2m 55s
linux clang --disable-ssl	2m 38s
linux gcc test -300	9m 26s
linux gcc test 301-600	6m 33s
linux gcc test 601- unstable	6m 29s
linux clang test sanitizers -300	12m 49s
linux clang test sanitizers 301-600	9m 12s
linux clang test sanitizers 601- unstable	11m 17s
linux gcc test -ljemalloc -300	9m 53s
linux gcc test -ljemalloc 301-600	6m 31s
linux gcc test -ljemalloc 601- unstable	6m 50s
linux gcc system-test-dpdk dpdk -100	17m 9s
linux gcc system-test-dpdk dpdk 101- unstable	14m 29s
linux gcc system-test-userspace -100	15m 54s
linux gcc system-test-userspace 101- unstable	13m 27s
linux gcc system-test -100	18m 29s
linux gcc system-test 101- unstable	17m 23s
linux clang system-test sanitizers -100	20m 12s
linux clang system-test sanitizers 101- unstable	19m 18s
linux x86 gcc --disable-ssl	2m 36s
	3h 55m 49s

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