Cache and restore the frame-format, so that test_set_frame_format doesn't modify the default, as required for TestInferiorAssert.py.

- Also fixes this test case to set/verify a non-default frame-format and explain the intent.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@191378 91177308-0d34-0410-b5e6-96231b3b80d8