This required a bit of manual manipulation. These tests dynamically
fetch generated IDL files, e.g.:
https://github.com/web-platform-tests/wpt/blob/master/interfaces/streams.idl
Our WPT importer is not able to detect the IDL files that need to be
imported, so dom.idl and streams.idl was copied over manually. Further,
idlharness.js would create URLs of the form "file://interfaces/dom.idl".
So idlharness.js was adapted to create a URL relative to the test file.
