Reduce CI test job execution time by 2/3

This MR brings the time needed to run the test_facet_grid.py test module down by about 70%. This reduces test execution time from about 6 minutes to about 2 minutes, yay.

This is achieved by:

• combining some test cases such that multiple features are tested at the same time
• reducing the amount of test data

The test coverage was not affected by these changes.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped

Related issues

Closes #271 (closed)