Remove usage of QiskitTestCase (qiskit-community#1285)

This change removes the dependence on `QiskitTestCase`, replacing it
with a direct dependence on `unittest.TestCase` and
`testtools.TestCase`.

As with `QiskitTestCase`, the ability to run the tests based either on
`unittest.TestCase` or `testtools.TestCase` (a `unittest.TestCase`
subclass) is preserved. For qiskit-experiments, the ability is actually
restored because the timeout feature added in

[qiskit-community#1246](qiskit-community#1246)
had introduced a hard dependence on `testtools`.

Specific changes:

* Add `testtools` and `fixtures` to `requirements-dev.txt` as required
  test dependencies.
* Use `QE_USE_TESTTOOLS` environment variable to control whether tests
  are based on `testtools.TestCase` rather than checking if `testtools`
is installed.
* Remove some checks for test writing best practices. `QiskitTestCase`
  used extra code to ensure that `setUp` and other test class methods
always called their parents and that those methods are not called from
individual tests.  `testtools.TestCase` does these checks as well. Since
qiskit-experiments always uses `testtools` in CI, it can rely on
`testtools` for these checks and just not do them for the alternate
`unittest` execution.
* Generate `QiskitExperimentsTestCase` from a `create_base_test_case`
  function. This function allows the base test class to be generated
based on either `testtools.TestCase` or `unittest.TestCase` so that the
`unittest` variant can be tested for regressions even when the
`testtools` variant is enabled.

Closes
[qiskit-community#1282](qiskit-community#1282).

CONTRIBUTING.md

@@ -121,8 +121,8 @@ Note that tests will fail automatically if they do not finish execution within 6
 
 #### STDOUT/STDERR and logging capture
 
-When running tests in parallel using `stestr` either via tox, the Makefile (`make
-test_ci`), or in CI, we set the env variable `QISKIT_TEST_CAPTURE_STREAMS`, which will
+When running tests in parallel using `stestr` either via tox
+or in CI, we set the env variable `QISKIT_TEST_CAPTURE_STREAMS`, which will
 capture any text written to stdout, stderr, and log messages and add them as attachments
 to the tests run so output can be associated with the test case it originated from.
 However, if you run tests with `stestr` outside of these mechanisms, by default the
@@ -134,6 +134,18 @@ stdlib unittest runner, a similar result can be accomplished by using the
 [`--buffer`](https://docs.python.org/3/library/unittest.html#command-line-options)
 option (e.g. `python -m unittest discover --buffer ./test/python`).
 
+#### Other testing related settings
+
+The test code defines some environment variables that may occasionally be useful to set:
+
++ `TEST_TIMEOUT`: An integer representing the maximum time a test can take
+  before it is considered a failure.
++ `QE_USE_TESTTOOLS`: Set this variable to `FALSE`, `0`, or `NO` to have the
+  tests use `unittest.TestCase` as the base class. Otherwise, the default is
+`testtools.TestCase` which is an extension of `unittest.TestCase`. In some
+situations, a developer may wish to use a workflow that is not compatible with
+the `testtools` extensions.
+
 ### Code style
 
 The qiskit-experiments repository uses `black` for code formatting and style and

requirements-dev.txt

@@ -1,6 +1,8 @@
 qiskit-terra>=0.45.0
 black~=22.0
+fixtures
 stestr
+testtools
 pylint~=3.0.2
 astroid~=3.0.1  # Must be kept aligned to what pylint wants
 jinja2==3.0.3