In the WarnUnusedCliUnused test, the whole CMakeCache.txt was removed in the
clean stage to trigger the same CMake warning in re-builds.  This technique
worked only in the Makefile generators and the test was limited to these.

Now only the variable of interest is removed from the cache by using a
`unset(UNUSED_CLI_VARIABLE CACHE)` statement in the CMakeLists.txt file.
This makes the WarnUnusedCliUnused test run on all generators