Revert "Merge branch 'test-matrix-refactor' into 'master'"

This reverts merge request !207 (merged)

@fikl2 This change seems to have pretty consistently caused some crashes in Github CI:

• https://github.com/inducer/pytential/runs/882980671?check_suite_focus=true#step:3:942
• https://github.com/inducer/pytential/pull/20/checks?check_run_id=883180552#step:3:923

The logs even point to a test involved here:

[gw1] linux -- Python 3.8.4 /home/runner/work/pytential/pytential/.miniforge3/envs/testing/bin/python3
worker 'gw1' crashed while running "test/test_matrix.py::test_block_builder[<context factory for <pyopencl.Device 'pthread-Intel(R) Xeon(R) Platinum 8171M CPU @ 2.60GHz' on 'Portable Computing Language'>-scalar-1.0-p2p-3]"

This reverts that change while we investigate.