feat(test): allow running test executables on the device

This introduces a `labgrid_test` rule, which allows running a test executable on the device (`:inner` can be any `*_binary` target):

```
labgrid_test(
    name = "example",
    src = ":inner",
    platform = ":platform",
)
```

It integrates with Bazel XML reporting functionality by writing to `${XML_OUTPUT_FILE}` on the host, if a XML report is generated on the device.

I've left a few `FIXME`'s which will be addressed in follow-up MRs.