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/src/main.ts

@@ -414,6 +414,7 @@ modules must contain a "plugin" attribute. For more information check the plugin
         this.incrementPendingTags();
         this.decrementPendingTags();
         await this.scriptTagsPromise;
+        await this.interpreter._remote.pyscript_py._schedule_deferred_tasks();
     }
 
     // ================= registraton API ====================