`wait_for_idle` should raise if `apps` is not a list

[sed-i]

It is a common error to pass an str instead of List[str] to apps= in wait_for_idle, and as a result the test fails for timeout:

INFO     juju.model:model.py:2645 Waiting for model:
  p (missing)
  r (missing)
  o (missing)
  m (missing)
  e (missing)
  t (missing)
  h (missing)
  e (missing)
  u (missing)
  s (missing)

It would be handy if the test fails with a more obvious message.

python-libjuju/juju/model.py

Line 2536 in bd01785

:param apps (list[str]): Optional list of specific app names to wait on.