[wptrunner] Always write `results.html` and results JSON

This matches the behavior of `run_web_tests.py`, allows the partial
results to be viewed, and triggers archival on the next test run.
Without archival, `run_wpt_tests.py` fails to process most events and
pollutes the log with file "already exists" messages.

Fixed: 1475556
Change-Id: Ic2bbdabb8c48e97a411392f06bec0005a591185f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5187136
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Reviewed-by: Weizhong Xia <weizhong@google.com>
Cr-Commit-Position: refs/heads/main@{#1245642}