tests: Add broken animation to test suite

From data_S0626.tgz in https://bugzilla.gnome.org/show_bug.cgi?id=517195
jpeg: Assume that CMYK JPEGs are inverted CMYK

Photoshop seems to assume that CMYK JPEGs are in inverted CMYK format.
We now do the same. Hopefully it won't cause problems as most CMYK files
around are probably produced by that program.

https://bugzilla.gnome.org/show_bug.cgi?id=618096
tests: Move pixdata_equal() to helpers
tests: Add error reporting to pixdata_equal()
tests: Add a test for inverted CMYK JPEG files

https://bugzilla.gnome.org/show_bug.cgi?id=618096
io: Fix check if library is running inside build tree

The existing check was missing the length of the string so instead of reading
the end of the string it read the memory before it.

https://bugzilla.gnome.org/show_bug.cgi?id=733413
2.31.2