twister: pytest: Wrap iter_stdout

Added waiting for prompt instead of calling time.sleep,
to fix issue with longer setting up platforms with TF-M
enabled. Wrapped iter_stdout generator, to avoid issues
when generator is created and called multiple times
in test scenarios.
Fixes: #58747

Signed-off-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>