Flipped intermittent test failures from skip to XFAIL.

The following intermittently-failing tests have been flipped from
skip to XFAIL on some combo of Linux and MacOSX:

TestCallStopAndContinue.py (Linux, MacOSX)
TestCallWithTimeout.py (Linux)
TestConvenienceVariables.py (Linux)
TestStopHookMultipleThreads.py (Linux)

The following new tests have been marked XFAIL but are just
intermittently failing:

TestMultipleDebug.py (definitely intermittent on MacOSX, not sure I've seen
it pass yet on Linux)