Fix skipping reftests and add helper to skip ref tests. (#24798)

Reftests that were being skipped were still trying to run the reftest
snapshot code and report a result. Since this was async, the result
would sometimes come in while the next test was started and cause
"excessive responses from previous test".

This should fix flakes with several test_webgl tests.

Author: brendandahl

test/browser/webgl_draw_base_vertex_base_instance_test.c

@@ -70,7 +70,7 @@ int main() {
 
   if (!extAvailable) {
     EM_ASM({
-      fetch('http://localhost:8888/report_result?skipped:%20WEBGL_draw_instanced_base_vertex_base_instance%20is%20not%20supported!').then(() => window.close());
+      skipTest('WEBGL_draw_instanced_base_vertex_base_instance is not supported');
     });
     return 0;
   }

test/browser/webgl_multi_draw_test.c

@@ -60,7 +60,7 @@ int main()
 
   if (!extAvailable) {
     EM_ASM({
-      fetch("http://localhost:8888/report_result?skipped:%20WEBGL_multi_draw%20is%20not%20supported!").then(() => window.close());
+      skipTest('WEBGL_multi_draw is not supported');
     });
     return 0;
   }

test/browser_reporting.js

@@ -52,6 +52,10 @@ function reportStdoutToServer(message) {
   }
 }
 
+async function skipTest(message) {
+  await reportResultToServer(`skipped:${message}`);
+}
+
 function reportTopLevelError(e) {
   // MINIMAL_RUNTIME doesn't handle exit or call the below onExit handler
   // so we detect the exit by parsing the uncaught exception message.

test/canvas_animate_resize.c

@@ -126,8 +126,7 @@ int main()
   if (!ctx) {
     if (!emscripten_supports_offscreencanvas()) {
       EM_ASM({
-        fetch("http://localhost:8888/report_result?skipped:%20OffscreenCanvas%20is%20not%20supported!")
-        .then(() => window.close());
+        skipTest('OffscreenCanvas is not supported!');
       });
     }
     return 0;

test/common.py

@@ -2413,11 +2413,11 @@ def run_browser(self, html_file, expected=None, message=None, timeout=None, extr
           # the browser harness reported an error already, and sent a None to tell
           # us to also fail the test
           self.fail('browser harness error')
+        output = unquote(output)
         if output.startswith('/report_result?skipped:'):
           self.skipTest(unquote(output[len('/report_result?skipped:'):]).strip())
         else:
           # verify the result, and try again if we should do so
-          output = unquote(output)
           try:
             self.assertContained(expected, output)
           except self.failureException as e:

test/reftest.js

@@ -23,6 +23,7 @@ function reftestUnblock() {
 function doReftest() {
   if (reftestBlocked) return;
   if (doReftest.done) return;
+  if (reportResultToServer.reported) return;
   doReftest.done = true;
   var img = new Image();
   img.onload = () => {