ci(spanner): Create a new Sample Slow tests #10188

Job	Run time
dependencies (17)	1m 15s
javadoc	2m 33s
lint	1m 27s
units (11)	3m 49s
units (8)	3m 44s
units (17)	3m 23s
units (21)	3m 20s
units-with-multiplexed-session (11)	4m 5s
units-with-multiplexed-session (8)	4m 28s
units-with-multiplexed-session (17)	3m 59s
units-with-multiplexed-session (21)	3m 51s
windows	19m 2s
clirr	1m 17s
	56m 13s

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