With Python binding, it gets very easy to test new functions. I've been
wondering if we need C counterparts, but really since it's a GObject
Introspection binding, if a function succeeds there, it should also succeed in C
code.

For now, I'm testing a few of GimpPalette API. Not all of it yet. Also I test
both the direct C binding and PDB procedure since in some cases, one or the
other may not properly working. See #10885.