Ruby: Address test flakiness in google-cloud-firestore bulkwriter test

[aandreassa]

Example failure on ubuntu-latest, 3.2

https://github.com/googleapis/google-cloud-ruby/actions/runs/27338407074/job/80768535873

Finished in 144.498781s, 6.9412 runs/s, 44.8447 assertions/s.

  1) Failure:
Google::Cloud::Firestore::BulkWriter::mock_firestore::thread tests#test_0001_request threads tests [test/google/cloud/firestore/bulkwriter_test.rb:222]:
Expected 2 to be == 3.

1003 runs, 6480 assertions, 1 failures, 0 errors, 0 skips
[2026-06-11 10:27:12 ERROR]  Minitest failed!