Remove unnecessary libavutil/(avutil|common|internal).h inclusions

Some of these were made possible by moving several common macros to
libavutil/macros.h.

While just at it, also improve the other headers a bit.

Reviewed-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavutil/tests/color_utils.c

@@ -18,7 +18,9 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include <stdio.h>
 #include "libavutil/color_utils.c"
+#include "libavutil/macros.h"
 
 int main(int argc, char *argv[])
 {

libavutil/tests/pixelutils.c

@@ -16,6 +16,10 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include <stdio.h>
+
+#include "libavutil/internal.h"
+#include "libavutil/mem.h"
 #include "libavutil/pixelutils.c"
 
 #define W1 320