ci(spanner): Create a new Sample Slow tests #10190

Job	Run time
units (11)	3m 31s
units (17)	3m 24s
units-with-multiplexed-session (8)	4m 33s
units (21)	3m 19s
clirr	1m 15s
dependencies (17)	1m 20s
javadoc	2m 32s
lint	1m 24s
units (8)	3m 50s
units-with-multiplexed-session (11)	3m 49s
windows	7m 2s
units-with-multiplexed-session (17)	4m 19s
units-with-multiplexed-session (21)	3m 50s
	44m 8s

Provide feedback