Revert "Remove CTestTestfile.cmake when BUILD_TESTING is OFF"
This reverts !258 (merged). It breaks projects that never enable testing but create their own CTestTestfile.cmake manually instead.
System updates will be applied on October 23th between 0930 AM and 1200 PM, EDT (UTC-04:00). This site may have intermittent downtime during that time.
This reverts !258 (merged). It breaks projects that never enable testing but create their own CTestTestfile.cmake manually instead.