Unify setup and teardown methods of TestCases

[jakelishman]

Summary

The previous split was to avoid having testtools as a core testing
dependency, even though it was present in all CI tests. This eventually
led to more fracturing of the two classes, where various extra
functionality was added into FullQiskitTestCase, but not into
BasicQiskitTestCase (see #6746). This meant downstream consumers of
QiskitTestCase sometimes had to care about how Terra's tests were
configured (see Qiskit/qiskit-aer#1283), and running simple local tests
without setting the environment variable QISKIT_TEST_CAPTURE_STREAMS
would run with different configurations.

This unifies the two classes, such that QiskitTestCase is now the parent
class of FullQiskitTestCase. All non-testtools/non-fixtures related
additions are handled in QiskitTestCase, and FullQiskitTestCase adds on
the testtools-specific functionality. FullQiskitTestCase is still not
made a subclass of testtools.TestClass because the same issues mentioned
in #3982 are still present in testtools.

Whether to capture streams and whether to use the testtools machinery
are two separate concerns, with the availability of the former being
strictly dependent on the latter. We can bind to the "full"
testtools-based class provided testtools is available, and it internally
decides whether it should also add the stream-capturing mechanisms.

The shared functionality added to the setUp, setUpClass and tearDown
methods means that it is important all subclasses always call up the
stack with super(). testtools has always enforced this using custom
_run_setup() methods, but this is not available to us when it is not
being used as the runner. Instead, the parent class is modified after
creation, wrapping certain methods with additional checks, and the
init_subclass method is used to influence the creation of children,
to add similar tests when they are created.

Details and comments

Fix #6746

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[mtreinish]

Overall LGTM, I really like the approach here and the end state is much better than what was there before, thanks for doing this.

I have one inline concern about backwards compat for some of our downstream consumers. Honestly having this stuff in an exported interface is awful and we should deprecate the entire qiskit.test module and move things around (like having test.mock move to qiskit.providers or somewhere else) and everything else just lives in the tests. But we can save that for a follow up PR.

[jakelishman]

I mixed my two responses to the comments (see #6753 (comment)). To be honest, I don't especially mind exporting some testing utilities from qiskit.test, but it would be good to make a much clearer distinction between what's a utility intended to be shared by all Qiskit-family packages (and consequently is a public API with all the guarantees that implies), and what's specific to testing Terra's features (which we can consider private). But yeah, a different PR for that.

[jakelishman]

Ok @mtreinish I've reinstated the BaseQiskitTestCase and BasicQiskitTestCase names. The former is the base, and should only contain functionality that can be shared across all Qiskit-family packages. The latter is just an alias to QiskitTestCase, which is the lowest possible Terra-specific case. I haven't really attempted to make any real split yet though, since that's probably better handled in a different PR.

[mtreinish]

LGTM, thanks for the quick update. I'll open an issue about the deprecation of qiskit.test where we can discuss how exactly we want to do that.