Add runFixtureTestsWithoutExactASTMatch in parser test runner

[JLHwung]

Q	A
License	MIT

This PR introduces a small refactor over Babel parser fixture test runner. It extracts runFixtureTestsWithoutExactASTMatch so we can run the test routine to different Babel parser options.

Also fixed the issue that estree-throw will overwrite the fixtures when process.env.OVERWRITE is ture-ish.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/45694/

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[nicolo-ribaudo]

I edited the title a bit to respect GH's ui commit title max length.