It's ok for symlinks in the builddir to be broken.
We can't (easily) detect that they actually point to something that will
exist after make install.

With a typical kdesrc-build setup the test passed because it ignored the
builddir (which is outside the srcdir) completely.

In CI however the builddir is a subdir, so the recursive directory
traversal in the unittest ends up there...

bin/symlink works again. I created a broken symlink in the srcdir, it detected it.