t_glob.c: add test cases for hidden directory and file

The existing test code was geared towards every little bit of
performance. It even duplicated the file definitions in vfs_stat in order
to avoid a few strcmp calls. This made the test code fragile. Therefore,
vfs_stat has been rewritten completely to not duplicate any information
from the vfs.

In vfs_stat, the returned st_mode is now more realistic. It had been 0
before. The file mode is only logged when it makes sense. In the ENOENT
case it is not logged anymore.

The debug logging for opendir/closedir now logs the same pointer, so that
the corresponding calls can be matched easily. Failed vfs_opendir calls
are logged as well, to get a more complete picture of which callbacks are
called.

(rillig)