Run a single WPT test from a relative path

Fixes #4055
auto merge of #4063 : mttr/servo/test_wpt_from_relpath, r=mbrubeck

Fixes #4055 

This PR allows `./mach test-wpt` to be run  (from /path/to/servo) with a single relative path to some wpt test.

For example:
```
./mach test-wpt tests/wpt/web-platform-tests/dom/sometest.html
```
The argument `tests/wpt/web-platform-tests/dom/sometest.html` is passed on as `--include dom/sometest.html`.