Add simple utility for unit testing jscodeshift transform

[Daniel15]

Adds a simple utility to allow unit testing of jscodeshift transforms in a standard way, without having to write a bunch of boilerplate code. This is loosely based off the technique used in react-codemod, with some changes. The main change is that where possible it doesn't depend on any global environment/configuration:

• It's in an actual module rather than a global function called "test". It took me so long to work out where the test function was coming from in the js-codemod repo 😛
• Assumes the transform is in the parent directory of the test (eg. transform is at foo/bar.js, test is at foo/__tests__/bar-test.js)
• Does not use a .js suffix for the test fixtures (input and expected output). This is mainly so they don't accidentally get picked up as unit tests by Jest, or as code that should be bundled with the app.

Also included an example of how to use it, using a simple transform (the default example from AST Explorer).

[cpojer]

this should be a lazy require inside of runTest. Jest resets the module registry after each test and we had issues previously where two copies of ast-types was present.

cc @pieterv

[Daniel15]

Oooh, good point. Will fix.

[Daniel15]

Updated.

[fkling]

Awesome! Would you mind mentioning this in the readme somewhere?

[Daniel15]

Sure :) I was thinking about it a bit, and I think I'll move the test fixtures to a separate directory to keep them separate from the unit test files themselves.

[Daniel15]

Updated to add a section to the readme.

[fkling]

Thanks, this is great!

[Daniel15]

@fkling - It would be useful to mention this in the release notes for v0.3.20, mainly so people know that this is only included from that version onwards.

[fkling]

Whoops, yes of course! Sorry for forgetting to add this!

[Daniel15]

No worries 👍