test(firestore): fix flaky BulkWriter thread tests #109264

Job	Run time
CI (ubuntu-latest, 3.3, --test)	3m 27s
CI (ubuntu-latest, 3.2, --test)	3m 28s
CI (macos-latest, 4.0, --test)	3m 6s
CI (ubuntu-latest, 3.4, --test)	3m 35s
CI (ubuntu-latest, 4.0, --test)	3m 14s
CI (windows-latest, 4.0, --test)	5m 21s
CI (ubuntu-latest, 4.0, --rubocop-toplevel --rubocop --build --yard)	56s
	23m 7s

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