Allow conversion from element of quotient ring to cover ring

[user202729]

As in the title.

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[user202729]

It's natural that this conversion ought to work (conversion need not be canonical or natural or being a homomorphism or anything).
But we defines

That leads to the following default implementation of element containment testing:

.. NOTE::

    `x \in P` holds if and only if the test ``x==P(x)`` does not
    raise an error and evaluates as true.

and there is a test that says the parent generator is not in the child ring.
E.g. there was a test QQ[x].0 not in QQ[x].quotient(x^2+1).

On the other hand we have 1 in GF(3) returns True just fine. Same for 4 in GF(3).

We cannot change the implementation of __contains__ either, because then sqrt(-1) in QQbar also returns False. (sqrt(-1) has parent SR which wraps the pyobject I (element of quadratic number field), and there's no coercion from SR to QQbar)

Therefore I decide to just modify the tests accordingly.