Fix potential import error from test runner

Since Qiskit#3982 merged we've been using the python libraries fixtures and
testtools to construct a custom test results handler that can handle
attachments for parallel test execution. While this is only used by the
test runner the imports could cause an issue for isntalled copies of
terra trying to use the fake backends. Since the requirements-dev.txt
isn't installed for regular installs if a user were to import the
fake_backends with a plain install this would result in an ImportError
because fixtures and/or testtools wasn't installed. Since these aren't
really requirements for terra, and are only required to run tests this
commit wraps the imports in a try block so no error will be raised
unless it's actually a test run.

At the same time fixtures and testtools
are explicitly added to the requirements-dev.txt list. They are
implicitly installed because they're requirements of stestr. But having
them explicitly in the requirements-dev.txt list makes this dependency
for running tests explicit, especially in cases where a user isn't using
stestr as their test runner.

qiskit/test/base.py

@@ -27,10 +27,13 @@
 import sys
 import unittest
 from unittest.util import safe_repr
-
-import fixtures
-from testtools.compat import advance_iterator
-from testtools import content
+try:
+    import fixtures
+    from testtools.compat import advance_iterator
+    from testtools import content
+    HAS_FIXTURES = True
+except ImportError:
+    HAS_FIXTURES = False
 
 from .runtest import RunTest, MultipleExceptions
 from .utils import Path, _AssertNoLogsContext
@@ -86,6 +89,10 @@ class QiskitTestCase(unittest.TestCase):
 
     def __init__(self, *args, **kwargs):
         """Construct a TestCase."""
+        if not HAS_FIXTURES:
+            raise ImportError(
+                "Test runner requirements are missing, install "
+                "requirements-dev.txt and run tests again.")
         super(QiskitTestCase, self).__init__(*args, **kwargs)
         self.__RunTest = self.run_tests_with
         self._reset()

qiskit/test/runtest.py

@@ -28,7 +28,11 @@
 
 import sys
 
-from testtools.testresult import ExtendedToOriginalDecorator
+try:
+    from testtools.testresult import ExtendedToOriginalDecorator
+    HAS_TESTTOOLS = True
+except ImportError:
+    HAS_TESTTOOLS = False
 
 
 class MultipleExceptions(Exception):
@@ -75,6 +79,10 @@ def __init__(self, case, handlers=None, last_resort=None):
                 raised - aborting the test run as this is inside the runner
                 machinery rather than the confined context of the test.
         """
+        if not HAS_TESTTOOLS:
+            raise ImportError(
+                'Test runner requirements are missing, install '
+                'requirements-dev.txt before running tests')
         self.case = case
         self.handlers = handlers or []
         self.exception_caught = object()

requirements-dev.txt

@@ -8,6 +8,8 @@ pycodestyle
 pydot
 astroid==2.3.3
 pylint==2.4.4
+fixtures>=3.0.0
+testtools>=2.2.0
 stestr>=2.0.0
 PyGithub
 wheel