Enable worker tests (#1757)

This PR re-enables tests on `worker`s. Highlights:
 
* by default, each test is run twice: the main thread version uses `<script type="py">`, the worker version automatically turn the tags into `<script type="py" worker>`

* you can tweak the settings per-class by using the `@with_execution_thread` decorator. In particular, `@with_execution_thread(None)` is for those tests which don't care about it (e.g., `test_py_config.py`)

* inside each class, there might be some test which should be run only in the main thread (because it doesn't make sense to test it in a worker). For those, I introduced the `@only_main` decorator

* we might introduce `@only_worker` in the future, if needed

* `@skip_worker` is for those tests which currently pass on main but not on workers. These are meant to be temporary, and eventually they should all be fixed
 
During the process, I tweaked/improved/fixed/deleted some of the existing tests. Some of them were at risk of being flaky and I made them more robust, others depended on some very precise implementation detail, and I made them more generic (for example, `test_image_renders_correctly` relied on pillow to render an image with a very specific string of bytes, and it broke due to the recent upgrade to pyodide 0.24.1)
 
I also renamed all the skip messages to start with `NEXT`, so that they are easier to grep.

pyscript.core/tests/integration/test_shadow_root.py

@@ -4,8 +4,7 @@ from .support import PyScriptTest
 
 
 class TestShadowRoot(PyScriptTest):
-    # @skip_worker("FIXME: js.document")
-    @pytest.mark.skip("FIXME: Element interface is gone. Replace with PyDom")
+    @pytest.mark.skip("NEXT: Element interface is gone. Replace with PyDom")
     def test_reachable_shadow_root(self):
         self.pyscript_run(
             r"""