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)

llvm-svn: 212762
diff --git a/lldb/test/functionalities/stop-hook/multiple_threads/TestStopHookMultipleThreads.py b/lldb/test/functionalities/stop-hook/multiple_threads/TestStopHookMultipleThreads.py
index 86410e4..7fd553a 100644
--- a/lldb/test/functionalities/stop-hook/multiple_threads/TestStopHookMultipleThreads.py
+++ b/lldb/test/functionalities/stop-hook/multiple_threads/TestStopHookMultipleThreads.py
@@ -21,7 +21,7 @@
         self.stop_hook_multiple_threads()
 
     @dwarf_test
-    @skipIfLinux # llvm.org/pr15037 stop hooks sometimes fail to fire on Linux (cannot mark XFAIL, sometimes it passes)
+    @expectedFailureLinux("llvm.org/pr15037") # stop hooks sometimes fail to fire on Linux
     def test_stop_hook_multiple_threads_with_dwarf(self):
         """Test that lldb stop-hook works for multiple threads."""
         self.buildDwarf(dictionary=self.d)