spanner: Cleanup TestMaxOpenedSession
spanner: Cleanup TestMaxOpenedSession

Replace the 1 second waits in the test case for:
1. A shorter 10ms wait for timeout.
2. A semaphore for destroying the session handle.

Change-Id: I7f9343c3164956971f79ab34d436f02e5a628c41
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/47613
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Shanika Kuruppu <382bd9679e6cb7e2135c45a8cd1be9e02cd75f23@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

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