spanner: Unflake TestTakeFromWriteQueue
spanner: Unflake TestTakeFromWriteQueue

TestTakeFromWriteQueue inspected the internal data structure of
the session pool without locking the mutex that protects it.
This caused a data race if the health checker or maintainer also
tried to alter the state of the internal data structure at the
same time.

Fixes #1283.

Change-Id: Ib0e6d2d4807cb52fb2bfeed76d7bda57c7f63c04
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/45710
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go