chore(spanner): unflake unit tests #10334

Job	Run time
clirr	1m 7s
dependencies (17)	1m 23s
javadoc	2m 53s
lint	1m 9s
units (11)	2m 43s
units (17)	2m 57s
units (8)	3m 12s
units (21)	3m 6s
units-with-multiplexed-session (11)	3m 14s
units-with-multiplexed-session (17)	3m 15s
units-with-multiplexed-session (8)	3m 15s
units-with-multiplexed-session (21)	3m 10s
windows	6m 52s
	38m 16s

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