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 61bc9091) 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.

gstreamer/gst-plugins-base!94
8 jobs for 1-5-1 in 36 minutes and 4 seconds (queued for 10 minutes and 48 seconds)
Status Job ID Name Coverage
  Preparation
passed #126586
gst indent

00:01:20

passed #126585
manifest

00:00:59

 
  Build
passed #126587
build fedora x86_64

00:05:02

 
  Test
passed #126588
check fedora

00:03:41

 
  Full Builds
passed #126589
build android arm64 api28

00:02:54

passed #126591
build cerbero cross win32

00:09:19

passed #126592
build cerbero cross win64

00:06:46

passed #126590
build cerbero fedora x86_64

00:06:00