Marked TestObjCMethods failing tests XFAIL, fixed cleanup code, removed now-passing expected failure markers.

All tests matching '-p TestObjCMethods' now are marked correctly for MacOSX, and some
error classes have been removed in cleanup code looking for files that
might not exist due to previous failure.

See http://llvm.org/bugs/show_bug.cgi?id=20267


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212650 91177308-0d34-0410-b5e6-96231b3b80d8