Add TestCase assertion methods for testing checks

This patch adds assertCheckPasses and assertCheckFails methods to make
it easier to write tests for more complicated checks that we want to
add.

We rely on checks being generators at this point, meaning this will not
work on flake8 class based plugins.

Change-Id: I37ecec271006084211e37c4fc9c89d71df01dc97

hacking/tests/__init__.py

@@ -43,3 +43,16 @@ class TestCase(testtools.TestCase):
         if os.environ.get('OS_STDERR_CAPTURE') in _TRUE_VALUES:
             stderr = self.useFixture(fixtures.StringStream('stderr')).stream
             self.useFixture(fixtures.MonkeyPatch('sys.stderr', stderr))
+
+    def assertCheckFails(self, check_func, *args, **kwargs):
+        if not list(check_func(*args, **kwargs)):
+            raise AssertionError("Check %s did not fail." %
+                                 check_func.__name__)
+
+    def assertCheckPasses(self, check_func, *args, **kwargs):
+        try:
+            self.assertCheckFails(check_func, *args, **kwargs)
+        except AssertionError:
+            return
+        else:
+            raise AssertionError("Check %s failed." % check_func.__name__)