[DynInst_API:] RFC: unify the dyninst and testsuite repos

[Josh Stone <jistone@xxxxxxxxxx>]

How would folks feel about pulling the dyninst testsuite back into the
main dyninst repository?

I mentioned in a conversation about CI on github[1].  Having them in
separate repos makes it harder to configure a lot of those CI tools.
Any breaking API changes have to be pushed separately, inevitably
causing a failed build in the interim.
[1] https://github.com/dyninst/dyninst/pull/22

I think it would be much nicer for these to evolve in lockstep.

It should be very easy to import it back with git-subtree, preserving
all commit history.  In fact, since it all used to be one repo, a lot of
the git objects will even still be identical, so the metadata won't even
grow much to bring it back.

For the start, we can still keep them as independent CMake projects,
with the testsuite requiring an installed dyninst root.  Eventually, I
think even this should be merged though, as I don't see a lot of use for
the testsuite to act independently.

It will also increase the source release tarball size from git-archive,
but I think this isn't a big deal.  DyninstAPI-9.1.0.tgz was ~8.7MB, and
Testsuite-9.1.0.tgz was ~1.8MB, a relatively small addition.  Both could
be made smaller with bzip2 or even xz, if anyone cares.