tests: Call openpty (if available) without using dlsym

The SONAME of libutil varies between architectures, so the logic to find
the SONAME of libutil was only correct for native builds (Linux on
Linux), not for cross-builds. The regular expression was also not
sufficiently broad to match the SONAME used on the alpha architecture,
which is apparently libutil.so.1.1.

Instead of screen-scraping the output of ldconfig and using that to
dlopen the library that contains openpty, it seems more reliable to
emit a link-time reference to openpty and let the linker do its job.
It's also less code.

Bug-Debian: https://bugs.debian.org/1007946
Signed-off-by: Simon McVittie <smcv@collabora.com>
13 jobs for wip/smcv/simplify-openpty-linking in 22 minutes and 31 seconds (queued for 1 minute and 9 seconds)