chore(spanner): unflake unit tests #10328

Job	Run time
units (11)	3m 7s
units (11)	3m 1s
units (8)	3m 12s
units (8)	3m 5s
units (17)	2m 54s
units (17)	2m 56s
units-with-multiplexed-session (11)	3m 41s
units-with-multiplexed-session (11)	3m 21s
units (21)	3m 20s
units (21)	2m 36s
units-with-multiplexed-session (17)	3m 24s
units-with-multiplexed-session (17)	3m 18s
units-with-multiplexed-session (8)	3m 20s
units-with-multiplexed-session (8)	3m 19s
units-with-multiplexed-session (21)	3m 34s
units-with-multiplexed-session (21)	3m 19s
windows	18m 16s
windows	7m 0s
clirr	1m 45s
clirr	1m 7s
dependencies (17)	1m 35s
dependencies (17)	1m 20s
javadoc	2m 59s
javadoc	2m 36s
lint	1m 13s
lint	1m 27s
	1h 30m 45s

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