Make wpt use mach-style logging by default.
Don't ignore errors in wpt/run.sh.
Make wpt run as part of make check.
Stop importing html5lib in wpt/run.sh.

It is only required when updating MANIFEST.json, which we don't do as part of
the run.
auto merge of #2250 : Ms2ger/servo/run-wpt, r=jdm