Make manifest path and expectation root independent

[lukebjerring]

Currently, the MANIFEST.json is assumed to share a root with metadata (.inf) files. We should add support for passing in a different expectation root than the manifest path (--metadata-root).

This would allow us to build + utilize a root which pulls from the latest wpt.fyi results (re-using the expectation behaviour to detect regressions), for #7475

[jgraham]

Probably the right way to do this is to change metadata_root to expectation_root and manifest_path everywhere and for compat set manifest_path=os.path.join(metadata_root, "MANIFEST.json") and expectation_root=metadata_root if the manifest root is explicitly passed in in a command line argument (we can make the argument hidden so it's only legacy scripts that do this).

[alijuma]

This has priority:roadmap. Is there any work planned for this quarter?

[gsnedders]

fixed by #9676.