gh-90473: Misc test fixes for WASI (GH-93218)

* ``sys.executable`` is not set
* WASI does not support subprocess
* ``pwd`` module is not available
* WASI checks ``open`` syscall flags more strict, needs r, w, rw flag.
* ``umask`` is not available
* ``/dev/null`` may not be accessible
(cherry picked from commit 1f134e96ba994bea3aaea533d4a558df3c5cc3c0)

Co-authored-by: Christian Heimes <christian@python.org>