plug-ins: add file-plug-in testing framework

Copied over from the original separate work in:
https://gitlab.gnome.org/Wormnest/gimp-file-plugin-tests
After that further improved and changed and added more file format
tests.

Added meson.build files to integrate it in our build, but we do not
install it for releases.

po-python/POTFILES.skip

@@ -49,6 +49,7 @@ plug-ins/maze
 plug-ins/metadata
 plug-ins/pagecurl
 plug-ins/print
+plug-ins/python/tests
 plug-ins/screenshot
 plug-ins/script-fu
 plug-ins/selection-to-path