Add more self tests

[Mark Wielaard <mjw at redhat dot com>]

I added some helpers to test-subr.sh to do self tests and use that
for the elflint-self test and two new self tests for readelf and nm.
That last one would have caught the issue Petr recently fixed.
The other tests on top of the new dwarfstrings patches have been
reworked similarly. The new C++ testcase has been much simplified,
it now only does the one thing (base type encoding checking) that
it was originally written for. But it can be extended for other
C++ constructs we want to check. There is now also a binary testfile
for it. I didn't change the license, I think we always have to provide
source for all binary test files we ship. I did add a NOTE to the
source file to make clear that if you want to change it you either
have to regenerate the binaries or make a new source test case.