[tests] option to show warnings even if successfully compiled

Examples on how to test compile all aircrafts/targets in your current conf.xml:
with parallel compilation and showing full output during compilation
```
J=AUTO prove tests/examples -v
```
only showing full compile output if there has been an error, if there were warnings only print those
```
SHOW_WARNINGS_ONLY=1 prove test/examples
```
with parallel compilation and treating all warnings as errors:
```
J=AUTO USER_CFLAGS=-Werror prove tests/examples
```