This whole thing changes it so that the JS and the UI both use
rustc's own path printing to handle the impl IDs. This results in
the format changing a little bit; full paths are used in spots
where they aren't strictly necessary, and the path sometimes uses
generics where the old system used the trait's own name, but it
shouldn't matter since the orphan rules will prevent it anyway.
Some tests will delete their output directory before starting.
The output directory is based on the test names.
If one test is the prefix of another test, then when that test
starts, it could try to delete the output directory of the other
test with the longer path.
