chore(spanner): unflake unit tests #10367

Job	Run time
units-with-multiplexed-session (8)	7m 49s
units (11)	1m 21s
units (17)	1m 18s
units (21)	1m 35s
dependencies (17)	1m 19s
clirr	1m 15s
javadoc	2m 44s
lint	1m 22s
units (8)	1m 17s
units-with-multiplexed-session (11)	1m 14s
units-with-multiplexed-session (17)	1m 16s
windows	2m 21s
units-with-multiplexed-session (21)	1m 21s
	26m 12s

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