spanner: background goroutines are left running

[egonelbre]

Tracking issue for https://github.com/googleapis/google-cloud-go/pull/10095

Currently the session client workers outlive the client, which may cause use of closed or released resources. This often can show up as flaky tests. Quite possibly something like https://github.com/googleapis/google-cloud-go/pull/9758#issuecomment-2091000887

Similarly, the tests should wait for the server completely shutdown, otherwise there's a similar problem.