tests: Do not leave ppm files around

When doing a --compare-images the test writes out a test image
ppm file and then uses Imagemagick to compare that against the
golden image. If they match, remove the test ppm.
diff --git a/tests/xgltestframework.cpp b/tests/xgltestframework.cpp
index 18379ff..12e4a01 100644
--- a/tests/xgltestframework.cpp
+++ b/tests/xgltestframework.cpp
@@ -302,6 +302,15 @@
     DestroyMagickWand(magick_wand_1);
     DestroyMagickWand(magick_wand_2);
     MagickWandTerminus();
+
+    if (differenz == 0.0)
+    {
+        /*
+         * If test image and golden image match, we do not need to
+         * keep around the test image.
+         */
+        remove(testimage);
+    }
 }
 
 void XglTestFramework::Show(const char *comment, XglImage *image)