Use nested parameterization in test_sdl_key. NFC (emscripten-core#20738)

I'm tying this out as an experiment.  I think it improves things in
several ways:

- Increases precision of tests
- Aids in debugging (makes run just one code path much easier)
- Cleaner test output as tests no longer have to `print()` each time
  they go around the loop (which was a crutch to help identify which
  iteration failed).

Now this single test has 8 different configurations (the 9th test
here, test_sdl_key_proxy, is actually a different test):

```
$ ./test/runner browser.test_sdl_key*
Test suites:
['test_browser']
Running test_browser: (9 tests)
(checking sanity from test runner)
shared:INFO: (Emscripten: Running sanity checks)
[Browser harness server on process 858049]
common:INFO: Using default system browser

Running the browser tests. Make sure the browser allows popups from localhost.

test_sdl_key (test_browser.browser.test_sdl_key) ... ok
test_sdl_key_asyncify (test_browser.browser.test_sdl_key_asyncify) ... ok
test_sdl_key_asyncify_eventhandler (test_browser.browser.test_sdl_key_asyncify_eventhandler) ... ok
test_sdl_key_delay (test_browser.browser.test_sdl_key_delay) ... ok
test_sdl_key_delay_asyncify (test_browser.browser.test_sdl_key_delay_asyncify) ... ok
test_sdl_key_delay_asyncify_eventhandler (test_browser.browser.test_sdl_key_delay_asyncify_eventhandler) ... ok
test_sdl_key_delay_eventhandler (test_browser.browser.test_sdl_key_delay_eventhandler) ... ok
test_sdl_key_eventhandler (test_browser.browser.test_sdl_key_eventhandler) ... ok
test_sdl_key_proxy (test_browser.browser.test_sdl_key_proxy) ... ok
[Browser harness server terminated]
```

test/test_browser.py

@@ -903,44 +903,46 @@ def test_glgears_proxy_jstarget(self):
   def test_sdl_canvas_alpha(self):
     # N.B. On Linux with Intel integrated graphics cards, this test needs Firefox 49 or newer.
     # See https://github.com/emscripten-core/emscripten/issues/4069.
-    create_file('flag_0.js', '''
-      Module['arguments'] = ['-0'];
-    ''')
+    create_file('flag_0.js', "Module['arguments'] = ['-0'];")
 
     self.btest('test_sdl_canvas_alpha.c', args=['-lSDL', '-lGL'], reference='browser/test_sdl_canvas_alpha.png', reference_slack=12)
     self.btest('test_sdl_canvas_alpha.c', args=['--pre-js', 'flag_0.js', '-lSDL', '-lGL'], reference='browser/test_sdl_canvas_alpha_flag_0.png', reference_slack=12)
 
   @parameterized({
-    '': ([False],),
+    '': ([],),
+    'eventhandler': (['-DTEST_EMSCRIPTEN_SDL_SETEVENTHANDLER'],),
+  })
+  @parameterized({
+    '': ([],),
+    'asyncify': (['-DTEST_SLEEP', '-sASSERTIONS', '-sSAFE_HEAP', '-sASYNCIFY'],),
+  })
+  @parameterized({
+    '': (False,),
     'delay': (True,)
   })
-  def test_sdl_key(self, delay):
-    for defines in [
-      [],
-      ['-DTEST_EMSCRIPTEN_SDL_SETEVENTHANDLER']
-    ]:
-      for async_ in [
-        [],
-        ['-DTEST_SLEEP', '-sASSERTIONS', '-sSAFE_HEAP', '-sASYNCIFY']
-      ]:
-        print(delay, defines, async_)
-
-        create_file('pre.js', '''
-          function keydown(c) {
-           %s
-            var event = new KeyboardEvent("keydown", { 'keyCode': c, 'charCode': c, 'view': window, 'bubbles': true, 'cancelable': true });
-            document.dispatchEvent(event);
-           %s
-          }
+  def test_sdl_key(self, delay, async_, defines):
+    if delay:
+      settimeout_start = 'setTimeout(function() {'
+      settimeout_end = '}, 1);'
+    else:
+      settimeout_start = ''
+      settimeout_end = ''
+    create_file('pre.js', '''
+      function keydown(c) {
+       %s
+        var event = new KeyboardEvent("keydown", { 'keyCode': c, 'charCode': c, 'view': window, 'bubbles': true, 'cancelable': true });
+        document.dispatchEvent(event);
+       %s
+      }
 
-          function keyup(c) {
-           %s
-            var event = new KeyboardEvent("keyup", { 'keyCode': c, 'charCode': c, 'view': window, 'bubbles': true, 'cancelable': true });
-            document.dispatchEvent(event);
-           %s
-          }
-        ''' % ('setTimeout(function() {' if delay else '', '}, 1);' if delay else '', 'setTimeout(function() {' if delay else '', '}, 1);' if delay else ''))
-        self.btest_exit('test_sdl_key.c', 223092870, args=defines + async_ + ['--pre-js=pre.js', '-lSDL', '-lGL'])
+      function keyup(c) {
+       %s
+        var event = new KeyboardEvent("keyup", { 'keyCode': c, 'charCode': c, 'view': window, 'bubbles': true, 'cancelable': true });
+        document.dispatchEvent(event);
+       %s
+      }
+    ''' % (settimeout_start, settimeout_end, settimeout_start, settimeout_end))
+    self.btest_exit('test_sdl_key.c', 223092870, args=defines + async_ + ['--pre-js=pre.js', '-lSDL', '-lGL'])
 
   def test_sdl_key_proxy(self):
     create_file('pre.js', '''