Fix running tests without `--develop` flag

• setup.cfg: Don't collect tests from src directory

  Generally we don't have any tests in the src directory so we don't need to collect anything from that directory. The only exception to this are the sourcetests, but they add their own collection hook so they are not affected by this.

  This fixes an issue where pytest gets confused upon finding two different conftest.py modules - one in the src directory and one in the virtual environment.

  Fixes #1121 (closed).

• .gitlab-ci.yml: Add CI job to run tests without --develop flag

  This is to prevent us from breaking *-nocover environments, and in general, ensure that we can run tests correctly without --develop flag.