Supporting junit XML report in rust_test

Currently `rust_test` does not support generating junit xml report out-of-the-box.

To support this for all rust test target, a workaround is with the `--run_under` flag and a script that writes the xml report from stdout to $XML_OUTPUT_FILE:

```
#!/bin/bash
$@ -Zunstable-options --format=junit --report-time | tee $XML_OUTPUT_FILE
```

The main downside is having to declare the script as a `data` dependency for every rust test target.

It would be nice to have built-in support for this in rust_test as an option, would it be sensible to add this to the rust test launcher?