py: unittests.py: Add weakref helpers and use del

Add helpers to check if a weakref or a list of weakrefs is alive or
dead.

Also use 'del' for local variables instead of setting the variable to
None. This makes debugging the test easier as the locals will be gone
from locals() dict.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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