Avoid creating nmake inference rule problems

**Problem**

rostest's `add_ros_test` calls on windows would run through the cmake ok, but when hitting nmake, it would stop short with the error message:

```
makefile(1035) : fatal error U1086: inference rule cannot have dependents
Stop.
```

In the `Makefile`, you would find a target like:

```
.run_tests_rostest_rostest_test_hztest0.test: cmake_check_build_system
	$(MAKE) -f CMakeFiles\Makefile2 /nologo -$(MAKEFLAGS) .run_tests_rostest_rostest_test_hztest0.test
```

Nmake would read this like an inference rule (aka `.c.obj:`) and abort when it saw the dependency (http://msdn.microsoft.com/en-us/library/d8k6a82k.aspx).

**Solution**

Is there a special reason the *hidden* targets are using a dot? I've used underscore here.