Add base class for sources

and test the all sources adhere to the set contract.

Also standardise the source (triggers to come) class names
to NameSource.

This will make it easier to do more sources in the future and also
add the possibility of loading sources dynamically.

Co-Authored-By: Gregory Haynes <greg@greghaynes.net>

Change-Id: I15b32013904f60873601dd7cc8fce3c158787de4
diff --git a/tests/base.py b/tests/base.py
index 7cd9de3..67de802 100755
--- a/tests/base.py
+++ b/tests/base.py
@@ -500,7 +500,7 @@
         return ret
 
 
-class FakeGerritSource(zuul.source.gerrit.Gerrit):
+class FakeGerritSource(zuul.source.gerrit.GerritSource):
     name = 'gerrit'
 
     def __init__(self, upstream_root, *args):