Rewrite manifest.py to make including local changes safer (not very backwards compatible)
Allow running tests from an external server root.

This is needed for supporting use cases where testsuites outside
wpt are run using the server in a wpt checkout. It also changes the
manifest format in a way that causes the file path to be derived from
the url unless otherwise specified.
fixup review issues
Remove unrelated change.
Merge pull request #1281 from w3c/jgraham/serve_alternate_root

Allow running tests from a path outside the test root.