tests: Add tests for the thumbnail verification code in GIO

This code was added for use by the G_FILE_ATTRIBUTE_THUMBNAIL_IS_VALID
file attribute, but may end up being used elsewhere (e.g. in GVfs) as well.
As it’s dealing with untrusted external files, and the non-trivial PNG file
format, this commit adds several test cases to cover valid and invalid PNG
files.

The security model for the thumbnail verification code is that the user’s
cache directory is untrusted, and potentially any PNG file which is passed
to the verifier has been manipulated arbitrarily by an attacker.

This is a follow-up to commit fe7069749fe39a006985ec266260a3c02ee8c855.

https://bugzilla.gnome.org/show_bug.cgi?id=709898