Fix test_async and test_stdio_handling (#1319)

Resolves pyscript#1313 and pyscript#1314. On top of pyscript#1318.

The point of these tests is to define the execution order of Tasks
that are scheduled in <py-script> tags: first all the py-script tags
are executed and their related lifecycle events. Once all of this
is done, we schedule any enqueued tasks.

To delay the execution of these tasks, we use a custom event loop for
pyExec with this defer behavior. Until schedule_deferred_tasks is called,
we defer tasks started by user code. schedule_deferred_tasks starts all 
deferred user tasks and switches to immediately scheduling any further
user tasks.

pyscriptjs/tests/integration/test_01_basic.py

@@ -192,16 +192,15 @@ class TestBasic(PyScriptTest):
         assert self.console.log.lines[-1] == "hello from foo"
 
     def test_py_script_src_not_found(self):
-        self.pyscript_run(
-            """
-            <py-script src="foo.py"></py-script>
-            """
-        )
+        with pytest.raises(JsErrors) as exc:
+            self.pyscript_run(
+                """
+                <py-script src="foo.py"></py-script>
+                """
+            )
         assert self.PY_COMPLETE in self.console.log.lines
 
         assert "Failed to load resource" in self.console.error.lines[0]
-        with pytest.raises(JsErrors) as exc:
-            self.check_js_errors()
 
         error_msgs = str(exc.value)