Tests: Add option to disable FindPackageModeMakefileTest

This test does not work in all environments, so add an option to
disable it.

Tests/FindPackageModeMakefileTest/CMakeLists.txt

 
 
-if(UNIX  AND  "${CMAKE_GENERATOR}" MATCHES "Makefile")
+if(UNIX  AND  "${CMAKE_GENERATOR}" MATCHES "Makefile" AND
+    NOT CMake_TEST_NO_FindPackageModeMakefileTest)
 
   # Test whether the make is GNU make, and only add the test in this case,
   # since the configured makefile in this test uses $(shell ...), which