Don't run the WarnUnusedCliUnused test on Windows.

Even with the NMake Makefiles generator, it is not able to properly
clean up after the test.

 http://open.cdash.org/testDetails.php?test=193424220&build=2934501

 Internal cmake changing into directory: C:/Dashboards/My Tests/CMake-nmake10-x64-continuous/Tests/WarnUnusedCliUnused
 Error: cmake execution failed
 CMake Error: Error: generator : NMake Makefiles
 Does not match the generator used previously: Visual Studio 10
 Either remove the CMakeCache.txt file or choose a different binary directory.
Merge topic 'suppress-unused-cli-with-value-in-cache' into next

6a6cda5 Don't run the WarnUnusedCliUnused test on Windows.