Add etstool.py support for generating ctraits.c coverage

On Linux and macOS, getting test coverage information for ctraits.c via `gcov` is as simple as doing

```
export CFLAGS="-coverage"
python etstool.py install --editable
python etstool.py test
```

You should then find `ctraits.gcno` and `ctraits.gcda` files in your build directory. The first is generated at install time. The second contains the actual coverage data, and is generated at test time.