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