Minor test runner improvemenst
- rework the way SBDebugger.SetAsync() is used to avoid side effects (reset original value at TearDownHook)
- refactor expectedFailureClang (and add expectedFailureGcc decorator)
- mark TestChangeValueAPI.py as expectedFailureGcc due to PR-15039



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@175523 91177308-0d34-0410-b5e6-96231b3b80d8