Fix /contenttype/guess test

Fix /contenttype/guess test

After fixing bug 674452 this test case now reliably fails, as "ABC abc" is text
and definitively not PowerPoint. It previously worked as g_content_type_guess()
was reading beyond the boundary of the data due to specifying -1 as data
length.

Update that test case to expect a PO template instead, and add two more with a
definitive PO template syntax and some binary data. We do not currently have a
MIME magic for PowerPoint, so we cannot actually detect it with certainty, but
at least make sure that the returned MIME type is correct.

https://bugzilla.gnome.org/show_bug.cgi?id=678941