Investigate build parallelization in travis

https://docs.travis-ci.com/user/speeding-up-the-build/

The unit tests, integration tests and UI tests could be parallelized.

Parallelizing the integration tests with each other may require copying the database file to a tmp location, otherwise the DB lock may be a problem. However, we run these tests in DB read-only mode so it may not require a change. They will need a unique data-dir.

Start by reading the linked Travis CI build-speeding documentation and locating the repository's Travis configuration and commands for unit, integration, and UI tests. Check whether read-only integration tests can share the database safely; completion means the test groups run in parallel, integration jobs have unique data directories, and the full build remains reliable.

Written by the indexing model from the issue text.