spanner: fix races in tests
spanner: fix races in tests

Some reads of sessionPool state were not protected by a mutex.

Change-Id: I82881dd2cd01e6af40ba85a380f2720453ed76b1
Reviewed-on: https://code-review.googlesource.com/15350
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Sarah Adams <2b342d1c32491b4e7587e4a0e291a5f0f95d4167@google.com>

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