Add testing documentation (#1760)

Contributes towards https://github.com/spec-first/connexion/issues/1531

connexion/apps/abstract.py

@@ -267,7 +267,8 @@ class AbstractApp:
         """
 
     def test_client(self, **kwargs):
-        """Creates a test client for this application."""
+        """Creates a test client for this application. The keywords arguments passed in are
+        passed to the ``StarletteClient``."""
         return TestClient(self, **kwargs)
 
     def run(self, import_string: str = None, **kwargs):

docs/testing.rst

@@ -0,0 +1,50 @@
+Testing
+=======
+
+test_client
+-----------
+
+Connexion exposes a ``test_client`` which you can use to make requests against your
+Connexion application during tests.
+
+.. code-block:: python
+
+    def test_homepage():
+        app = ...  # Set up app
+        kwarg = {...}
+        with app.test_client(**kwargs) as client:
+            response = client.get("/")
+            assert response.status_code == 200
+
+
+The passed in keywords used to create a `Starlette` ``TestClient`` which is then returned.
+
+For more information, please check the `Starlette documentation`_.
+
+.. _Starlette documentation: https://www.starlette.io/testclient/
+
+TestContext
+-----------
+
+To have access to the :doc:`context` variables during tests, you can use the :class:`.TestContext`
+provided by Connexion.
+
+.. code-block:: python
+
+    from unittest.mock import MagicMock
+
+    from connexion.context import operation
+    from connexion.testing import TestContext
+
+
+    def get_method():
+        """Function called within TestContext you can access the context variables here."""
+        return operation.method
+
+    def test():
+        operation = MagicMock(name="operation")
+        operation.method = "post"
+        with TestContext(operation=operation):
+            assert get_method() == "post
+
+If you don't pass in a certain context variable, the `TestContext` will generate a dummy one.