ci(spanner): Create a new Sample Slow tests #10189

Job	Run time
units (11)	3m 42s
units (8)	3m 42s
units-with-multiplexed-session (11)	3m 51s
units (17)	3m 28s
units-with-multiplexed-session (8)	3m 57s
units-with-multiplexed-session (17)	3m 50s
units (21)	3m 23s
units-with-multiplexed-session (21)	3m 44s
clirr	1m 15s
dependencies (17)	1m 17s
javadoc	2m 27s
lint	1m 13s
windows	19m 26s
	55m 15s

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