chore(spanner): unflake unit tests #10330

Job	Run time
units (11)	2m 49s
units (17)	2m 59s
units-with-multiplexed-session (11)	3m 38s
units-with-multiplexed-session (17)	3m 27s
units-with-multiplexed-session (21)	3m 8s
units (21)	3m 11s
clirr	1m 18s
dependencies (17)	1m 17s
javadoc	2m 35s
lint	1m 23s
units (8)	3m 5s
units-with-multiplexed-session (8)	3m 32s
windows	8m 35s
	40m 57s

Provide feedback