function getJestUnitTestFunction(
  testEntry /*: TestEntry */,
) /*: (title: string, callback: () => void) => void */ {
  switch (testEntry.filter) {
    case 'focus':
      return it.only;
    case 'skip':
      return it.skip;
    default:
      return it;
  }
}
Given a test config's "filter" status, decides whether we should run it with
jest's it/fit/xit function.