Fix memory leaks in test cases

We must not forget to call gdFontCacheShutdown() when we have used
gdImageStringFT() and friends.

tests/fontconfig/basic.c

@@ -27,6 +27,7 @@ int main()
     gdTestAssertMsg(error == NULL, "%s", error);
 
     gdImageDestroy(im);
+    gdFontCacheShutdown();
 
     return gdNumFailures();
 }

tests/freetype/bug00132.c

@@ -34,5 +34,6 @@ int main()
 		}
 	}
 	gdImageDestroy(im);
+	gdFontCacheShutdown();
 	return error;
 }

tests/gdimagestringft/gdimagestringft_bbox.c

@@ -73,6 +73,7 @@ int main()
 	fclose(fp);
 done:
 	gdImageDestroy(im);
+	gdFontCacheShutdown();
 	free(path);
 	return error;
 }

tests/gdimagestringftex/gdimagestringftex_returnfontpathname.c

@@ -18,6 +18,7 @@ int main()
 		return 2;
 	}
 	gdFree(strex.fontpath);
+	gdFontCacheShutdown();
 	free(path);
 	return 0;
 }