tests: Add try-syscall helper

This exercises various syscalls. It's heavily based on the one from
<https://github.com/containers/bubblewrap/pull/459

>, but with the
addition of a mode to output the numeric values of various expected
errno codes, which are not otherwise available to shell scripts.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 4ce251882c488953ca6e3734f00c5dbe2e1e3e7a)