add `google-cloud-spanner` to bulk-release once system tests pass

systems tests for `google-cloud-spanner` failed on a recent bulk release: https://github.com/googleapis/google-cloud-python/pull/18293. As a result, it was moved to individual release: https://github.com/googleapis/google-cloud-python/pull/18335.

See (build #7104): https://fusion2.corp.google.com/ci;prev=s/kokoro/prod:cloud-devrel%2Fclient-libraries%2Fpython%2Fgoogleapis%2Fgoogle-cloud-python%2Fpresubmit%2Fsystem/activity/042e3116-27da-4743-83c9-684ca8b11ce8/summary?q=google-cloud-python&s=p

Fix the system tests for `google-cloud-spanner`, and add it back to the bulk-release.

Start by reviewing pull requests 18293 and 18335 and the system-test failure from build #7104 for google-cloud-spanner. Reproduce the system tests, identify and fix the failure, then verify that google-cloud-spanner passes and is included in the bulk release again.

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