Skip TestCallStopAndContinue and TestCallThatRestarts on Darwin.

These are failing intermittently.

See http://llvm.org/bugs/show_bug.cgi?id=19246 for TestCallThatRestarts.  Also applies to Linux.
See http://llvm.org/bugs/show_bug.cgi?id=20274 for TestCallStopAndContinue.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212660 91177308-0d34-0410-b5e6-96231b3b80d8