In this test, it was possible for browser2 to finish loading _before_ the
BrowserTestUtils.browserLoaded function we were awaiting on for browser1 had
yielded back to the test, meaning that by the time we got to run
BrowserTestUtils.browserLoaded(browser2), the browser was already done loading,
and browserLoaded would be waiting for a page load that would never occur.
MozReview-Commit-ID: EagrRrAAdJP