All `wpt` subcommands should support non-standard test roots

[jonathan-j-lee]

Some wpt subcommands like tests-affected assume that the tests are located somewhere relative to the tooling source file, e.g.

wpt/tools/wpt/testfiles.py

Lines 28 to 29 in c5e9248

	here = os.path.dirname(__file__)
	wpt_root = os.path.abspath(os.path.join(here, os.pardir, os.pardir))

wpt/tools/wpt/testfiles.py

Line 249 in c5e9248

interface_dir = os.path.join(wpt_root, 'interfaces')

This is problematic for vendors like Chromium where:

• The tests are stored at //third_party/blink/web_tests/external/wpt/, but
• tools/ is stored separately in //third_party/wpt_tools/wpt/ for auditing and vulnerability scanning.

To support this structure, all wpt subcommands that interact with the repo should support an overridable --tests-root option. Some subcommands have already implemented this, such as:

• wpt run --tests
• wpt lint --repo-root