tests: use GPOINTER_TO_INT to avoid warnings with mingw

tests: use GPOINTER_TO_INT to avoid warnings with mingw

New casts to avoid the the warnings mentioned below. While at it, move
some existing casts (introduced at 61bc9091894062b9) to use
GPOINTER_TO_INT too.

[458/673] Compiling C object 'tests/check/7d01337@@libs_video@exe/libs_video.c.obj'.
../tests/check/libs/video.c: In function 'fourcc_get_size':
../tests/check/libs/video.c:160:10: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
   return (unsigned long) p->endptr;
          ^

In file included from ../tests/check/libs/video.c:32:
../tests/check/libs/video.c: In function 'test_video_formats':
../tests/check/libs/video.c:563:39: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
   fail_unless_equals_int (size, (unsigned long) paintinfo.endptr);
   ^

And more.

https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/merge_requests/94