Rename test_libpng_with_pthreads (#18041)

This is a crazy experiment to diagnose flake on the windows builder
(by trying to ensure that this test doesn't run simultaneously with
test_libpng)

Author: dschuff

test/test_other.py

@@ -1958,7 +1958,7 @@ def test_libpng(self):
     self.assertContained('libpng passes test', output)
 
   @node_pthreads
-  def test_libpng_with_pthreads(self):
+  def test_zzz_libpng_with_pthreads(self):
     shutil.copyfile(test_file('third_party/libpng/pngtest.png'), 'pngtest.png')
     self.emcc(test_file('third_party/libpng/pngtest.c'), ['--embed-file', 'pngtest.png', '-sUSE_LIBPNG', '-sUSE_PTHREADS'], output_filename='a.out.js')
     output = self.run_js('a.out.js')