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_stdio_handling.py

@@ -1,5 +1,3 @@
-import pytest
-
 from .support import PyScriptTest
 
 
@@ -100,7 +98,6 @@ class TestOutputHandling(PyScriptTest):
 
         self.assert_no_banners()
 
-    @pytest.mark.xfail(reason="fails after introducing synclink, fix me soon!")
     def test_targeted_stdio_async(self):
         # Test the behavior of stdio capture in async contexts
         self.pyscript_run(
@@ -149,7 +146,6 @@ class TestOutputHandling(PyScriptTest):
 
         self.assert_no_banners()
 
-    @pytest.mark.xfail(reason="fails after introducing synclink, fix me soon!")
     def test_targeted_stdio_interleaved(self):
         # Test that synchronous writes to stdout are placed correctly, even
         # While interleaved with scheduling coroutines in the same tag