This is a quick and dirty workaround for issue #3928. Basically, `cargo test` is deleting `./target/servo`, which is clearly not ideal if we want to do anything with servo after running the unit tests. This PR makes sure to rebuild after running `./mach test-unit`.
I'm not familiar enough with cargo yet to know why it's doing this or what better alternatives there are to fixing this. Having to rebuild afterwards feels pretty ugly to me, but my rationalization right now is that the time it takes to build is negligible in comparison to the time it takes to run the tests. Ideally, this should be something we could take care of in Cargo.toml, but again, I'm new to this (and the documentation seems less than helpful from what I can tell so far).
I won't be available for the rest of the day, so if anyone has suggestions, or wants to wait for a better solution, I'll get back to it tomorrow probably. Otherwise, this PR at least makes `./mach test` work properly, so there's that.
When running commands through Registrar.dispatch, mach does not behave
in the same way it would as if it were running through the command line.
Defaults normally provided through a combination of @CommandArgument and
argparse magic are ignored.
I have some ideas as to how to fix this, but until then, this will allow
`./mach test` to run through test-unit properly.
There are none, but the contenttest program fails with:
```
task '<main>' panicked at 'Required option 'source-dir' missing.', ../../tests/contenttest.rs:48
```
Minor change, added unit test filter to components so that ./mach test-unit [test-filter] works in line with documentation.
I'd personally like to also make filters on the components as well as the tests. This would change the interface (probably to ./mach test-unit [component-filter] [test-filter]), but change is NOT in this pull request.
Usage Example:
```
$ ./mach test tests/wpt/web-platform-tests/dom/interfaces.html
0:00.27 LOG: MainThread INFO Using 1 client processes
...
```
Note that while there is no functionality for `test-ref` to accept
individual files, `./mach test tests/ref` will still trigger `test-ref`.
