Rewrite indentation testing code

[axvr]

Benefits of new solution:

1. Faster (runs indentation tests in parallel)
   • On GitHub Actions, the tests now finish >10 seconds quicker, but we will see a greater benefit when I add more indentation tests (hopefully soon).
2. Less code
3. Adding new indentation test cases doesn't require editing the indent_test.clj Clojure file anymore
4. Fixed issues with the test vimrc so tests now work on newer Vim versions (previously they only worked on Vim 8.1 and below)
5. Can now set different config options per-test
6. Indentation tests now all pass on my computer

Now we're in a good position to start adding more indentation tests.