tests/elements/pip.py: Fixed test to declare pip source
diff --git a/tests/elements/pip/project.conf b/tests/elements/pip/project.conf
index f04cd98..4a9695d 100644
--- a/tests/elements/pip/project.conf
+++ b/tests/elements/pip/project.conf
@@ -7,6 +7,8 @@
 plugins:
 - origin: pip
   package-name: buildstream-plugins
+  sources:
+  - pip
   elements:
   - pip