Fix how options are merged with options from parent folder(s). It's much simpler than I had thought. If folder's options contains `plugins`, that overwrites `plugins` from parent options. They don't get merged.
Support overriding test fixtures in `update_fixtures.js` script.
Any files in `tasks/transform_conformance/overrides` are copied into Babel test fixtures. If `options.json` is overridden, then script runs Babel with updated options, to generate a new `output` file for the fixture.
Script to update fixtures for class properties transform was failing to transform fixtures where output is `output.mjs` (instead of `output.js`). This PR fixes that.
Add a NodeJS script which amends Babel's fixtures in place to remove transform plugins which we don't support from `options.json` files. Where options are changed from the original, the script runs Babel transform with the new options to regenerate the fixture `output.js` files.
Currently limited to transforming the fixtures for `babel-plugin-transform-class-properties` transform, but we can also enable it for other plugins if we wish in future, to get additional test coverage.
