Add Integration Tests

[agilgur5]

This is quite a big PR as it refactors a bunch of the test set-up to handle two different types of tests, as well as adds integration tests for tiny-invariant, rollup-plugin-postcss, styled-components, and regeneratorRuntime. Fixes #481

I initially tried to have the integration tests have their own package.json and node_modules, as the build-withConfig folder had before (and with a new CI workflow with working-directory set to the integration test dir), but this proved extremely confusing and very time-consuming to deal with. It basically requires that scripts, like tests, run from the integration test dir, as if they run from root, they won't read the node_modules (node scans upward, so from the integration test dir, it'll get the integration deps and the TSDX deps, but the opposite doesn't work). And trying to run scripts from there causes lots of inconsistencies all over the place, so I just scrapped that idea. Open to ideas on better integration testing structures, but the other option proved completely not worth the time, so don't want to spend a ton more time trying to get something else to work either.

Also found some integration failures all over the place while adding this, welp 😕 These include:

• --extractErrors removes invariant but doesn't replace it with ErrorProd or ErrorDev. And it leaves the import in, just importing nothing 😕
  • warning isn't changed what-so-ever
  • I still think this should be split out as its own Rollup or Babel plugin, it's fairly independent of TSDX, doesn't seem to be used much by users, and can benefit more than just TSDX
• rollup-plugin-postcss any PostCSS plugins like autoprefixer and cssnano work, but the name of the extracted CSS file is the same as the output, i.e. package-name.cjs.development.css
• babel-plugin-styled-components doesn't work, plugin order matters for it. See #543

[agilgur5]

Ok fixing the conflicts with #582 in yarn.lock was not easy 😕 . But done now, so this is ready to merge. Should go in before #525 , so need to review this soon

Realized I might be able to use yarn workspaces to have the integration-tests dir split out. Then TSDX can still be used (and as tsdx in scripts), but shared devDeps might not work correctly unless we put them in root similar to Lerna. I feel like that's really involved prior to having a monorepo (see #634, #635), but that might correctly describe the relationship between TSDX and its integrations; i.e. that it's a separate dependency.

Hmmmmmm... Then again, the relationship between integration tests and E2E tests is actually the same -- they both should be separate workspaces with tsdx used inside of them. So can hold off on this for a bit I guess, since will be refactoring the E2E to be unit tests eventually (and so might do the same for integration?)

[agilgur5]

• Rebased with #621 and added execWithCache everywhere like it has. This actually took a bit of effort due to the number of commits and renames to rebase against.
• Moved grep helper to utils/shell.js
• Moved util to utils/fixture.js (also took some effort due to renames / rewriting imports)
• Removed unused utils/psTree.js and its dependencies
• Moved utils-safePackageName.test.ts to a unit test dir
• Fixed a rebase issue I made where I accidentally merged two commits together that should be split
• Added a test that ensures Babel presets are merged, which was the original goal of #481, to add a regression test for the fix in #473