test(spanner): fix flaky TestReadWriteTransaction_ContextTimeoutDuringDuringCommit

[yfuruyama]

fixes #3914

As the following test message shows, it looks like the context is cancelled while the session is dynamically created for the transaction. To fix this issue, I used SessionPoolConfig to create a session explicitly and wait for the session to be created before we start testing the actual timeout for the commit.

2021/04/12 14:47:21 Integration tests skipped: GCLOUD_TESTS_GOLANG_PROJECT_ID is missing
=== RUN   TestReadWriteTransaction_ContextTimeoutDuringDuringCommit
=== PAUSE TestReadWriteTransaction_ContextTimeoutDuringDuringCommit
=== CONT  TestReadWriteTransaction_ContextTimeoutDuringDuringCommit
    client_test.go:1776: Error message mismatch:
        Got spanner: code = "DeadlineExceeded", desc = "timeout / context canceled during getting session.\nEnable SessionPoolConfig.TrackSessionHandles if you suspect a session leak to get more information about the checked out sessions."
        Want: spanner: code = "DeadlineExceeded", desc = "context deadline exceeded, transaction outcome unknown"
--- FAIL: TestReadWriteTransaction_ContextTimeoutDuringDuringCommit (0.21s)
FAIL
exit status 1
FAIL    cloud.google.com/go/spanner     0.496s

Also I fixed the typo in the test name: s/TestReadWriteTransaction_ContextTimeoutDuringDuringCommit/TestReadWriteTransaction_ContextTimeoutDuringCommit /.

[hengfengli]

LGTM.