Tests: Run MFC test only when explicitly enabled
Previously we used a complicated heuristic to decide whether or not to run the MFC test, but it sometimes decided incorrectly to run the test. Since that was first written, we have developed a convention for other tests to enable them via undocumented cache entries that are added only on machines known to meet the tests' requirements. Do that for MFC.
Backport: release
Edited by Brad King