[java-spanner] Slow integration tests should be improved to reduce build time

The slowest integration tests at the moment are:

```
[INFO] Tests run: 24, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 864.9 s -- in com.google.cloud.spanner.connection.it.ITDmlReturningTest
[INFO] Tests run: 6, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 358.9 s -- in com.google.cloud.spanner.it.ITAutogeneratedAdminClientTest
[INFO] Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 294.8 s -- in com.google.cloud.spanner.it.ITDatabaseRolePermissionTest
[INFO] Tests run: 7, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 292.6 s -- in com.google.cloud.spanner.it.ITDatabaseAdminTest
[INFO] Tests run: 29, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 462.7 s -- in com.google.cloud.spanner.connection.it.ITTransactionRetryTest
[INFO] Tests run: 12, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 323.4 s -- in com.google.cloud.spanner.it.ITClosedSessionTest
[INFO] Tests run: 8, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 246.4 s -- in com.google.cloud.spanner.it.ITBatchDmlTest
[INFO] Tests run: 10, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 234.8 s -- in com.google.cloud.spanner.it.ITCommitTimestampTest
```

The current overall build time for the normal integration tests are around 25 minutes.