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
g_content_type_guess() requires specifying a valid data length. Fixes a
segfault when running the test.
Also add an explicit check for this and return XDG_MIME_TYPE_UNKNOWN when
data_size is specified as -1, to avoid crashing.
https://bugzilla.gnome.org/show_bug.cgi?id=674452
