Modify logic to watch a finish of spec.

#### Problem:
My test page load some javascript files of reporter include `jasmine.junit_reporter.js`, but use other reporter only if set some flag.
If test page doesn't use `JUnitXmlReporter`, check logic doesn't work correctly to finish test runner.

#### Fix:
Add `JUnitXmlReporter.started_at` to check running state of `JUnitXmlReporter`.