Allow unit and selenium tests to run in parallel

[istride]

This PR allows test to be run in several processes at the same time, which substantially reduces the amount of time required to run a full test suite - especially Selenium tests.

Several improvements were required to make this possible:

• Run Selenium tests against a PostgreSQL database instead of SQLite
• Allow several test servers to be started by the Selenium test suite
• Introduce an env var to control parallelism

Even if the tests are not run in parallel, the Selenium tests should be more stable now due to the use of PostgreSQL - tests were failing randomly due to not being able to flush the SQLite database.

[istride]

Parallelism is not active for Selenium tests running in Github Actions because the standard runner only has two CPU cores available to it and the Selenium tests already require about 5 different processes to run just so that the tests can be run sequentially.