Using the wptrun infrastructure from upstream, it is now posible to
make it easy to run web-platform-tests in other browsers. The syntax
used is

mach wpt --product [chrome|servo|edge] [tests]

This will try to use the selected product; possibly prompting to
install dependencies like the WebDriver implementation. For servo if
the install isn't on the PATH then --binary can be used to point to
the actual location.

Because manifest metadata is kept in the same directory as expectation
data and we don't want to reuse Firefox expectation data for other
browsers, a new products subdirectory is introduced and added to the
ignore files. This will contain a subdirectory for each product into
which a copy of the test manifest is placed. It may also be used to
store any expectation data for the other products, in the same way as
testing/web-platform/meta.

MozReview-Commit-ID: 8fdCnha5t2F