ci(spanner): Create a new Sample Slow tests #10178

Job	Run time
javadoc	2m 37s
dependencies (17)	1m 24s
lint	1m 20s
units (11)	3m 31s
units (8)	3m 35s
units-with-multiplexed-session (11)	4m 13s
units (17)	3m 27s
units-with-multiplexed-session (17)	4m 3s
units-with-multiplexed-session (8)	3m 53s
units (21)	3m 27s
windows	7m 12s
units-with-multiplexed-session (21)	3m 44s
clirr	1m 8s
	43m 34s

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