chore(spanner): unflake unit tests #10368

Job	Run time
units (11)	3m 13s
units (11)	3m 13s
units (8)	3m 6s
units (8)	3m 6s
units (17)	3m 4s
units (17)	3m 4s
units (21)	3m 17s
units (21)	3m 17s
units-with-multiplexed-session (11)	3m 32s
units-with-multiplexed-session (11)	3m 32s
units-with-multiplexed-session (8)	3m 39s
units-with-multiplexed-session (8)	3m 39s
units-with-multiplexed-session (17)	3m 20s
units-with-multiplexed-session (17)	3m 20s
units-with-multiplexed-session (21)	3m 15s
units-with-multiplexed-session (21)	3m 38s
windows	6m 57s
windows	6m 57s
clirr	1m 24s
clirr	1m 24s
dependencies (17)	1m 55s
dependencies (17)	1m 55s
javadoc	2m 35s
javadoc	2m 35s
lint	1m 37s
lint	1m 37s
	1h 22m 11s

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