ci(spanner): Create a new Sample Slow tests #10179

Job	Run time
units (11)	3m 36s
units (17)	3m 50s
units-with-multiplexed-session (11)	4m 9s
units (21)	3m 32s
units-with-multiplexed-session (17)	3m 55s
units-with-multiplexed-session (21)	4m 1s
clirr	1m 12s
dependencies (17)	1m 18s
javadoc	2m 30s
lint	1m 16s
units (8)	3m 41s
units-with-multiplexed-session (8)	4m 9s
windows	7m 44s
	44m 53s

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