Disable cbmc-cpp tests in appveyor, which runs regression on windows.

All cbmc-cpp tests #include <assert.h> and cbmc cannot yet parse
Microsoft C++ headers.

Author: andreast271

appveyor.yml

@@ -83,6 +83,7 @@ test_script:
     rmdir /s /q cbmc\byte_update7
     rmdir /s /q cbmc\pipe1
     rmdir /s /q cbmc\unsigned___int128
+    rmdir /s /q cbmc-cpp
     rmdir /s /q cpp\Decltype1
     rmdir /s /q cpp\Decltype2
     rmdir /s /q cpp\Function_Overloading1