Remove the requirement to authenticate for /admin/server_version. (#5122)

This endpoint isn't much use for its intended purpose if you first need to get
yourself an admin's auth token.

I've restricted it to the `/_synapse/admin` path to make it a bit easier to
lock down for those concerned about exposing this information. I don't imagine
anyone is using it in anger currently.

tests/unittest.py

@@ -181,10 +181,7 @@ class HomeserverTestCase(TestCase):
             raise Exception("A homeserver wasn't returned, but %r" % (self.hs,))
 
         # Register the resources
-        self.resource = JsonResource(self.hs)
-
-        for servlet in self.servlets:
-            servlet(self.hs, self.resource)
+        self.resource = self.create_test_json_resource()
 
         from tests.rest.client.v1.utils import RestHelper
 
@@ -230,6 +227,23 @@ class HomeserverTestCase(TestCase):
         hs = self.setup_test_homeserver()
         return hs
 
+    def create_test_json_resource(self):
+        """
+        Create a test JsonResource, with the relevant servlets registerd to it
+
+        The default implementation calls each function in `servlets` to do the
+        registration.
+
+        Returns:
+            JsonResource:
+        """
+        resource = JsonResource(self.hs)
+
+        for servlet in self.servlets:
+            servlet(self.hs, resource)
+
+        return resource
+
     def default_config(self, name="test"):
         """
         Get a default HomeServer config object.