Fixture for writable Postgres engine to separate database-writable tests

[kathia-barahona]

When running parallelized tests, we need to keep in mind that database-writable tests might cause conflicts with other tests that are reading/writing the same table. Therefore, we have to separate them into a different xdist group (this happens only for Postgres). It would be nice to change the main Postgres engine fixture to read-only, and create a separate fixture for a writable engine.

[thijs-obj]

doh. I duplicated this issue in #771. Closing that one, just copy pasting the description from there here so we don't lose the ideas mentioned there.

text from 771:

Situation:

• Read-only tests can safely be parallelized.
• Tests that write to a database table need to be grouped, such that they are run consecutively.
  • We do this using the @pytest.mark.xdist_group annotation. But it is easy to forget a test

Idea: Use a read-only account for tests that are not marked as writing tests. That way they'll consistently fail instead of failing sometimes.

Future ideas:

• Split github tasks for writing and read-only tasks. Run read-only tests for all python versions in parallel
• Make more tests read-only, by pre-creating tables in database

[kathia-barahona]

This was already fixed when introducing unique_table_test_name fixture. https://github.com/objectiv/objectiv-analytics/blob/66dc7b7981b92f01652e77d38c3d11a6ab761e29/bach/tests/conftest.py#L100