[lldb][test][NFC] Refactor xfails in TestRequireHWBreakpoints.py

This file used a strange, multi-level setup where we skipped on
a function we used for xfailing. Let's not do that, just skip
the one test we care about.

Also added a comment to explain how this file works. The tests
*want* calls to fail when we ask for only hardware breaks
but have none to use.

If you don't know that, it all seems backwards.