Allow collecting coverage from test files

[efegurkan]

Do you want to request a feature or report a bug?
feature
What is the current behavior?

It is not possible to collect coverage if tests are in the same file with the source. Since should_instrument.js checks if file name matches testRegex or testMatches currently it is not possible to collect coverage from these files.

We have several tests similar to this example.

// sum.test.js

export function sum(a,b) {
  return a + b;
}

if (process.env.NODE_ENV === 'test') {
  test('sum', () => {
    expect(sum(1, 2)).toBe(3);
  });
}

What is the expected behavior?

It would be nice to have an option to check for coverage even if the file is a test file. Normally you wouldn't want to collect coverage from all of your test files.
Having and option like enforceCoverageRegex : ".*\\.test\\.js$" would allow you to collecting coverage from the files that are named in a certain way.

I am currently experimenting with the idea and i would like to come with a PR as well.

[efegurkan]

This has been merged with #5081.

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