test(spanner): fix MockAutoTaggingTest test environment pollution #19301

Job	Run time
owlbot-yaml	1m 5s
owlbot-py	1m 17s
gapic-bom	1m 46s
generate-readme	1m 11s
group_id_check_for_maps_libraries	9m 18s
gitignore_correctness	1m 6s
package_name_check	1m 6s
root-pom	1m 49s
	18m 38s

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