Modifies the Spring Data JPA sample to (also) run on the Spanner Emulator.
This was previously not possible, as the use of bit-reversed sequences in
Hibernate also requires the backend to support 'concurrent' transactions.
That is: When using bit-reversed sequences or other auto-generated primary
keys, Hibernate will do the following:
1. Start a transaction for the application and try to insert/update data
   in that transaction.
2. If a new auto-generated ID is required, start a separate transaction to
   generate a batch of identifiers.

The above would not work on previous versions of the emulator, as the
second transaction could not be started. The latest version of the
emulator supports this.

This change also adds a test run for the sample application to the
standard presubmit checks.

Fixes #2155