[tests] Move tests to a postgres environment

[mpsonntag]

Currently all tests are run using an h2 database. Unfortunately the h2 database sometimes behaves differently than the postgres database that is used in production, masking potential failures.

It would probably be best to gradually migrate to a postgres test environment and replace the h2 tests in the long run.

• Set up a postgres test database, ideally using docker, since this is as close to the production setup as we can get. Set up one use case test within the postgres test tree.
• Enable these tests on travis.
• Ensure the GCA-Web docker build is still working.
• Migrate the h2 tests to postgres tests.
• When all h2 tests are migrated remove the h2 test tree.
• Make sure the selenium tests work with the new test environment.