[test-runner] allow excluding test files

Negations in globs is a pretty hard syntax. We should support include/exclude patterns:

export default {
  files: {
    include: 'test/**/*.test.js',
    exclude: 'test/**/*.bar.test.js'
  }
}

[benjamind]

You can use glob negations, but the way we’re currently processing the glob strings means the negations are ineffective.

We could really use this capability to help separate our integration / unit tests.

[jordanaustin]

Yeah, we’re having the same problem.