Legacy test migration

[rykerfish]

There's a collection of old tests (tests/legacy_tests) that I've found useful to maintain consistent results when changing the code. The current format for these tests is pretty gross. I think these tests should be kept for more comprehensive coverage of end-to-end tests, especially since most of the current regression and parser tests don't test the grid code.

These legacy tests should be migrated to a pytest framework with a reduced amount of boilerplate in the setup for each test.