- better organized that way as the tests are somewhat independent of
  one another and previously even I was confused where to put different
  types of tests

- also refactor some test naming and some redundant usage of flags
  - especially for 0-config, where flags shouldn't be used

- and remove testEnvironment comment as it's already set in jest config

(optim): now that it's one fixture per file, we only need to copy the
fixture directory once per file, which should be a speed boost
- and sometimes may not need to re-run `tsdx build` when not necessary
- also the different test files get parallelized, which is also faster

- before: 53s avg on my machine, after: 37s avg on my machine
  - avg for running all tests
  - pretty BIG difference

- had removed some duplicate execs in the previous commit in order to
  speed up tests so that exec wouldn't need to be re-run
  - but this caused reproducibility issues, as if you change a test
    to `it.only` temporarily, it would fail hard as nothing would be
    exec'd if it weren't the first test
    - this uses an `execWithCache` function to instead naive "cache"
      the output and not re-run the same command twice-in-a-row
      - only works with sequential tests, but good enough for now
      - similar to a beforeAll() for a group of the same test, but no
        nesting or confusing variables in describe scope

- N.B. no need for stageName in cache function or something more
  complex because each stage is parallelized as multi-process