test(spanner): fix MockAutoTaggingTest test environment pollution #20978

Job	Run time
changes	1m 7s
windows	2m 28s
enforcer	19m 6s
gapic-libraries-bom	20m 22s
generation-config-check	1m 44s
bulk-filter	1m 6s
lint	3m 33s
units (8)	47s
units (11)	36s
units (21)	37s
units (25)	31s
units (17)	31s
split-clirr (java-spanner)	2m 31s
split-units (java-spanner, 8)	4m 37s
split-dependencies (java-spanner)	3m 55s
split-units (java-spanner, 17)	6m 43s
split-units (java-spanner, 11)	5m 5s
split-units (java-spanner, 25)	4m 53s
split-units (java-spanner, 21)	4m 25s
conditional-required-check	3s
	1h 24m 40s

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