fix Test_Packbits()

(64 characters is too low for the Win32 %TEMP% path)
should fix the appveyor build

src/tests/tests.c

@@ -151,7 +151,7 @@ int Test_CPC_compare_colors(void)
  */
 int Test_Packbits(void)
 {
-  char tempfilename[64];
+  char tempfilename[256];
   FILE * f;
   int i, j;
   long unpacked;