[wptrunner] Restore visibility of `results.html` on Linux

Currently, `run_wpt_tests.py` terminates the Xvfb process used for tests
after opening `results.html`. However, the browser opening
`results.html` effectively hides itself because it prefers to attach to
Xvfb instead of a real X server. Fix this by running the post-run tasks
in the opposite order.

This should also eliminate browser errors when Xvfb is closed. For
Firefox, this could leak messages like "Exiting due to channel errors."
into the log.

Bug: 1479856
Change-Id: If0f5ec8391d58baae35fc792e78c7b3902d04fe2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4854803
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Reviewed-by: Weizhong Xia <weizhong@google.com>
Cr-Commit-Position: refs/heads/main@{#1194395}