test: add test that dlopen()'s all our weak library deps once

test: add test that dlopen()'s all our weak library deps once

This test should ensure we notice if distros update shared libraries
that broke so name, and we still use the old soname.

(In contrast to what the commit summary says, this currently doesn#t
cover really all such deps, specifically xkbcommon and PCRE are missing,
since they currently aren't loaded from src/shared/. This is stuff to
fix later)