Avoid dirtying the source tree in breakpoint command tests
The paralelization patch exposed a bunch of cases where we were still
touching the source tree (as these tests were now stepping on each
others toes and being flaky).
This patch removes such issues from breakpoint command tests. Since the
only reason they were creating files was to indirectly test whether the
breakpoint commands got executed (and plumbing the full build tree path
to all places that needed it would be messy) I decided to modify the
tests to check for a different side effect instead: modification of a
global variable. This also makes the code simpler as checking the value
of the global variable is easier, and there is nothing to clean up.
As the tests aren't really doing anything debug-info related, I took the
opportunity to also mark them as NO_DEBUG_INFO_TESTCASEs.
Reviewers: jingham, aprantl
Differential Revision: https://reviews.llvm.org/D43464