add google-cloud-spanner to bulk-release once system tests pass #18341

systems tests for google-cloud-spanner failed on a recent bulk release: #18293. As a result, it was moved to individual release: #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.