Skip unit tests which require optional dependencies (#9031)

If we are lacking an optional dependency, skip the tests that rely on it.

tests/unittest.py

@@ -20,7 +20,7 @@ import hmac
 import inspect
 import logging
 import time
-from typing import Dict, Iterable, Optional, Tuple, Type, TypeVar, Union
+from typing import Callable, Dict, Iterable, Optional, Tuple, Type, TypeVar, Union
 
 from mock import Mock, patch
 
@@ -736,3 +736,29 @@ def override_config(extra_config):
         return func
 
     return decorator
+
+
+TV = TypeVar("TV")
+
+
+def skip_unless(condition: bool, reason: str) -> Callable[[TV], TV]:
+    """A test decorator which will skip the decorated test unless a condition is set
+
+    For example:
+
+    class MyTestCase(TestCase):
+        @skip_unless(HAS_FOO, "Cannot test without foo")
+        def test_foo(self):
+            ...
+
+    Args:
+        condition: If true, the test will be skipped
+        reason: the reason to give for skipping the test
+    """
+
+    def decorator(f: TV) -> TV:
+        if not condition:
+            f.skip = reason  # type: ignore
+        return f
+
+    return decorator