add async emterpreter browser test

tests/emterpreter_async.cpp

@@ -0,0 +1,18 @@
+#include<stdio.h>
+#include<emscripten.h>
+#include<assert.h>
+
+int main() {
+  // infinite main loop, turned async using emterpreter
+  int counter = 0;
+  while (1) {
+    printf("frame: %d\n", ++counter);
+    emscripten_sleep(100);
+    if (counter == 10) {
+      int result = 1;
+      REPORT_RESULT();
+      break;
+    }
+  }
+}
+

tests/test_browser.py

@@ -2444,3 +2444,6 @@ window.close = function() {
       args=['-s', 'USE_SDL=2', '-s', 'USE_SDL_IMAGE=2','--preload-file', 'screenshot.png', '-s', 'LEGACY_GL_EMULATION=1'],
       message='You should see an image with fog.')
 
+  def test_emterpreter_async(self):
+    self.btest('emterpreter_async.cpp', '1', args=['-s', 'EMTERPRETIFY=1', '-O3', '-g2'])
+