Move pngtest to tests/third_party/libpng/ (#12798)

These two files come directly the libpng distribution and
so belong in third_party.  I removed the local mod we had
done in favor of checking the existing output of the
upstream test code.

Fixes: #12795

Author: sbc100

tests/test_other.py

@@ -1446,9 +1446,10 @@ def test_sdl2_mixer_wav(self):
     building.emcc(path_from_root('tests', 'sdl2_mixer_wav.c'), ['-s', 'USE_SDL_MIXER=2'], output_filename='a.out.js')
 
   def test_libpng(self):
-    shutil.copyfile(path_from_root('tests', 'pngtest.png'), 'pngtest.png')
-    building.emcc(path_from_root('tests', 'pngtest.c'), ['--embed-file', 'pngtest.png', '-s', 'USE_LIBPNG=1'], output_filename='a.out.js')
-    self.assertContained('TESTS PASSED', self.run_process(config.JS_ENGINES[0] + ['a.out.js'], stdout=PIPE, stderr=PIPE).stdout)
+    shutil.copyfile(path_from_root('tests', 'third_party', 'libpng', 'pngtest.png'), 'pngtest.png')
+    building.emcc(path_from_root('tests', 'third_party', 'libpng', 'pngtest.c'), ['--embed-file', 'pngtest.png', '-s', 'USE_LIBPNG=1'], output_filename='a.out.js')
+    output = self.run_js('a.out.js')
+    self.assertContained('libpng passes test', output)
 
   def test_libjpeg(self):
     shutil.copyfile(path_from_root('tests', 'screenshot.jpg'), 'screenshot.jpg')

tests/pngtest.c renamed to tests/third_party/libpng/pngtest.c

@@ -2014,8 +2014,6 @@ main(int argc, char *argv[])
          (unsigned long) png_get_chunk_malloc_max(dummy_ptr));
    png_destroy_read_struct(&dummy_ptr, NULL, NULL);
 
-   if(ierror == 0)
-	   printf("TESTS PASSED\n");
    return (int)(ierror != 0);
 }
 #else