Find WPT files in Firefox repository

The Servo repository is now (mostly) vendored in the Firefox Mercurial
repository. For size and duplication reasons, the tests/wpt directory is
not included in the vendored copy.

This causes problems when running `mach` from the Firefox repository
because `mach` references pip requirements files and module search
paths from WPT.

This commit adds code to detect when Servo's mach is running from a
Firefox source tree and to resolve WPT paths to the Firefox location
if appropriate. This enables `mach` to "just work" when running
from the servo/ directory in the Firefox repository.

The file looked for to identify the Firefox repository is identical
to what Firefox's `mach` script uses.

A potential issue with using Firefox's WPT files is that they may be
different from those in the Servo repository and this could lead to
differences in behavior - possibly even an error when loading/running
`mach`. However, the behavior before this commit was that Servo's
`mach` never worked in the Firefox repository (due to missing WPT
files). And post-commit it does. So this seems like a "perfect is the
enemy of good" scenario.

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