Allow running |mach wpt| on one click loaners in order to run
web-platform-tests tests.
This implementation is just like the one for other testsuites using
thee packaged tests rather than the checkout that we get with wpt, at
least on Linux. That's also where the tests run from so it seems
reasonable for now. Moving to the checkout in the future could remove
some of the logic here by using a fake mozbuild environment so that
the testsuite itself doesn't have to implement anything much.
MozReview-Commit-ID: CaewrdjJ2ef
--HG--
extra : rebase_source : 491b8014d48f06ff5bd41b28cc985608981fbdf4
James Graham