add testing for `qiskit_ibm_runtime/fake_provider/`

[1ucian0]

With the move of the fake providers from qiskit to qiskit-ibm-runtime, the tests were not moved yet. This issue is to account for that, as the code has currently 0% coverage:

Tagging this as help wanted as a community member with some experience could move and adapt the tests from Qiskit.

[ElePT]

The tests were moved in https://github.com/Qiskit/qiskit-ibm-runtime/pull/1140, the base class PR (#1270) is a follow-up of this one. As far as I know, there are no base class unit tests to move. I am not sure why the coverage does not pick up the existing tests, I think it's because the main test is in integration and not unit tests.

[jyu00]

We should verify if tests exist

[kt474]

Tests already exist for fake_provider