test/osbuild: check the schema of sources

Add 'Source' to the list of module classes that will undergo basic
schema checks, i.e. verify that their embedded schema data is valid
JSON schema.

test/test_osbuild.py

@@ -81,7 +81,7 @@ class TestDescriptions(unittest.TestCase):
         index = osbuild.meta.Index(os.curdir)
 
         modules = []
-        for klass in ("Stage", "Assembler"):
+        for klass in ("Stage", "Assembler", "Source"):
             mods = index.list_modules_for_class(klass)
             modules += [(klass, module) for module in mods]