Skip results if testing was incomplete on unload

If a user navigates away from a page on which bandwidth is being tested
(i.e. before the file download probes are finished), the accuracy of
the results vary widely.

Therefore, don't save or report bw results if testing didn't complete
before user navigates off page.

A side-effect of this change is that bw testing will run on subsequent
pages until it finishes (which only requires that a user remain on a
page for 15s, so valid data should come quickly).

(cherry picked from commit 546889fa9f077b683c0dbdd39404e207a98760d0)

Signed-off-by: Philip Tellis <philip@lognormal.com>