function runJsUnitTests(testPrefix, browser) {
  return runBrowserTests(testPrefix, browser, 'tests');
}
Runs JSUnit tests.
@param {?string|?Array<string>} testPrefix Only tests that match the prefix
    will be returned. If null, all tests will run.
@param {string} browser The browser to run the unit tests on.
@return {!IThenable}