[test] Simplify post_manual_reftest. NFC

Rather than embedding `reftest.js` directly in the html, simply include
it via script src attribute.

test/test_browser.py

@@ -865,25 +865,23 @@ def post_manual_reftest(self):
     assert os.path.exists('reftest.js')
     html = read_file('test.html')
     html = html.replace('</body>', '''
+<script src="reftest.js"/>
 <script>
-function assert(x, y) { if (!x) throw 'assertion failed ' + y }
-%s
-
 var windowClose = window.close;
 window.close = () => {
   // wait for rafs to arrive and the screen to update before reftesting
-  setTimeout(function() {
+  setTimeout(() => {
     doReftest();
     setTimeout(windowClose, 5000);
   }, 1000);
 };
 </script>
-</body>''' % read_file('reftest.js'))
+</body>''')
     create_file('test.html', html)
 
   def test_sdl_canvas_proxy(self):
     create_file('data.txt', 'datum')
-    self.btest('test_sdl_canvas_proxy.c', reference='browser/test_sdl_canvas_proxy.png', args=['--proxy-to-worker', '--preload-file', 'data.txt', '-lSDL', '-lGL'], manual_reference=True, post_build=self.post_manual_reftest)
+    self.btest('test_sdl_canvas_proxy.c', reference='browser/test_sdl_canvas_proxy.png', args=['--proxy-to-worker', '--preload-file', 'data.txt', '-lSDL', '-lGL'])
 
   @requires_graphics_hardware
   def test_glgears_proxy_jstarget(self):
@@ -2115,7 +2113,7 @@ def test_cubegeom_pre3(self):
   def test_cubegeom(self, args):
     # proxy only in the simple, normal case (we can't trace GL calls when
     # proxied)
-    self.btest('third_party/cubegeom/cubegeom.c', reference='third_party/cubegeom/cubegeom.png', args=['-O2', '-g', '-sLEGACY_GL_EMULATION', '-lGL', '-lSDL'] + args, also_proxied=not args)
+    self.btest('third_party/cubegeom/cubegeom.c', reference='third_party/cubegeom/cubegeom.png', args=['-O2', '-g', '-sLEGACY_GL_EMULATION', '-lGL', '-lSDL'] + args)
 
   @no_wasm64('wasm64 + LEGACY_GL_EMULATION')
   @requires_graphics_hardware
@@ -3393,10 +3391,10 @@ def test_sdl2_canvas_write(self):
   def test_sdl2_gl_frames_swap(self):
     def post_build():
       self.post_manual_reftest()
-      html = read_file('test.html')
-      html2 = html.replace('''Module['postRun'] = doReftest;''', '') # we don't want the very first frame
-      assert html != html2
-      create_file('test.html', html2)
+      reftest = read_file('reftest.js')
+      reftest2 = reftest.replace("Module['postRun'] = doReftest;", '') # we don't want the very first frame
+      assert reftest != reftest2
+      create_file('reftest.js', reftest2)
     self.btest('test_sdl2_gl_frames_swap.c', reference='browser/test_sdl2_gl_frames_swap.png', args=['--proxy-to-worker', '-sGL_TESTING', '-sUSE_SDL=2'], manual_reference=True, post_build=post_build)
 
   @no_wasm64('SDL2 + wasm64')