Enhance the target parameter to also support testTarget like maven-compiler-plugin

```
The maven-compiler-plugin allows the config setting "testTarget", too. If it is 
empty, it falls back to "target".

forbidden should do the same, so also the system property is respected.
```

Original issue reported on code.google.com by `uwe.h.schindler` on 25 Oct 2013 at 9:10
