[test] Do not include "xmalloc.h" directly

Not needed, it's renamed and included by boilerplate directly.

test/buffer-diff.c

@@ -40,7 +40,6 @@
 
 #include "pdiff.h"
 #include "buffer-diff.h"
-#include "xmalloc.h"
 
 /* Don't allow any differences greater than this value, even if pdiff
  * claims that the images are identical */

test/cairo-test.c

@@ -52,7 +52,6 @@
 #include "cairo-test.h"
 
 #include "buffer-diff.h"
-#include "xmalloc.h"
 
 #ifdef _MSC_VER
 #include <crtdbg.h>

test/imagediff.c

@@ -35,7 +35,6 @@
 #include <errno.h>
 
 #include "buffer-diff.h"
-#include "xmalloc.h"
 
 static void
 _xunlink (const char *pathname)

test/pthread-show-text.c

@@ -30,7 +30,6 @@
 */
 
 #include "cairo-test.h"
-#include "xmalloc.h"
 
 #include <string.h>
 #include <stdlib.h>