Previously, we were assuming that the symlink created by
capsule-capture-libs would point to the `realpath()` of a library,
possibly with a `--link-target` prefix. However, it actually only
resolves the symlink representing the SONAME: it will resolve `libz.so.1`
to `libz.so.1.2.13`, but unlike Perl `abs_path()` or C `realpath()`,
it will not necessarily resolve symlinks in the directory hierarchy
leading up to that point.

For example, on a merged-/usr system like Debian >= 12, /lib is a
symbolic link to usr/lib. The realpath() of `libz.so.1` is something like
`/usr/lib/MULTIARCH/libz.so.1.2.13`, but because both `/lib/MULTIARCH`
and `/usr/lib/MULTIARCH` appear in `/etc/ld.so.conf.d`, it is undefined
whether capsule-capture-libs will output `/lib/MULTIARCH/libz.so.1.2.13`
or `/usr/lib/MULTIARCH/libz.so.1.2.13`.

Relax the expectations of this test so we only say that the symlink
points to some reasonable `$libdir`, followed by the `basename()` of the
`realpath()` of `libc.so.6`.

Signed-off-by: Simon McVittie <smcv@collabora.com>