test(spanner): fix MockAutoTaggingTest test environment pollution #2943

Job	Run time
filter	1m 5s
gapic-generator-java (8)	0s
sdk-platform-java integration	0s
unmanaged_dependency_check	0s
sdk-platform-java units (8)	0s
sdk-platform-java units	0s
dependency_analyzer_unit_test	0s
dependencies	0s
clirr	0s
sdk-platform-java integration (25)	0s
sdk-platform-java self-service clients	0s
gapic-generator-java-bom	0s
java8-compatibility	0s
	1m 5s

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