Support loading .mjs test files

[novemberborn]

AVA now recognizes .mjs test files, but refuses to load them. We should!

See the code here: https://github.com/avajs/ava/blob/54ff13036517e7dff3d95b7afe4e70254b1dbe7a/lib/worker/subprocess.js#L131:L137

Of course import() is only available on Node.js 13. This should fail gracefully on Node.js 10 and 12.

[novemberborn]

This is now available in AVA 3.3.0. Thank you @arlac77!