qunit: Improve testrunner logging for pending ajax

qunit: Improve testrunner logging for pending ajax

* Move `restoreWarnings()` in tearDown() to the mirrored location
  of related code in setUp().

* Ensure that accidentally calling `suppressWarnings()` twice will
  not wipe out the original reference indefinitely. If it was
  already set, subsequent calls should do nothing instead of
  overwriting them again so that recovery is still possible.

* Log all ajax requests logged during the test, not just the
  one currently still pending. This should avoid situations
  where we throw "Pending ajax requests" but no information
  is logged about which requests those might be.

Change-Id: I900ad98c4c8520bdd6ae00a24ac82272f3becfee