Unskip some tests, delete others (#1742)

Clean up a bit the testsuite and integration tests.
Highlights:

- Some of the @skipped tests just worked -- I unskipped them
- some worked after some small tweak to adapt to the new pyscript next
- some are still skipped, but I tweaked the skip message to be more precise and descriptive
- Moreover, I killed/removed the ones which no longer make sense in the context of pyscript next; in particular, I removed all the ones which tested Element (which is now gone) and the one which tested py-config features which are no longer needed (e.g., multiple interpreters).

The testsuite passes locally.

pyscript.core/tests/integration/test_event_handling.py

@@ -2,10 +2,6 @@ import pytest
 
 from .support import PyScriptTest
 
-pytest.skip(
-    reason="FIXME: @when decorator missing from pyscript", allow_module_level=True
-)
-
 
 class TestEventHandler(PyScriptTest):
     def test_when_decorator_with_event(self):