chore(spanner): unflake unit tests #10364

Job	Run time
dependencies (17)	1m 22s
javadoc	2m 38s
lint	1m 20s
units (11)	24m 5s
units (8)	1m 31s
units (17)	1m 31s
units-with-multiplexed-session (11)	24m 5s
units-with-multiplexed-session (8)	1m 14s
units (21)	1m 21s
units-with-multiplexed-session (17)	1m 23s
units-with-multiplexed-session (21)	1m 16s
windows	2m 27s
clirr	1m 6s
	1h 5m 19s

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