rtld: add some dlopen tests

dlopen_basic just tests that libthr.so can be dlopen()ed, which will
just serve as a sanity check that "libthr.so" is a thing that can be
dlopened in case we get a weird failure in dlopen_recursing.

dlopen_recursing tests a regression reported after the libsys split,
where some dlopen() may cause infinite recursion and a resulting crash.
This case is inspired by bdrewery's description of what seemed to be
causing his issue.

The corresponding fix landed in commit
968a18975ad ("rtld: ignore load_filtees() calls if we already [...]")

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D43859