Add target source packages for modules tests

This commit adds an inline test definition in the modules tests job to
add the source packages for the target repository.

Most of the modules tests require to fetch source packages and such a
repository is not available by default in the images.

Signed-off-by: Luis Araujo <luis.araujo@collabora.co.uk>

templates/group-modules-tpl.yaml

 {% include 'group-modules-standalone-tpl.yaml' %}
 
+  - test:
+      timeout:
+        minutes: 5
+      namespace: system
+      name: modules-tests
+      definitions:
+        - repository:
+            metadata:
+              name: add-target-sources
+              description: "Enable source packages for target"
+            run:
+              steps:
+                - grep target /etc/apt/sources.list | sed 's/^deb /deb-src /' > /etc/apt/sources.list.d/target-sources.list
+          from: inline
+          path: inline/add-target-sources.yaml
+          name: add-target-sources
+
   - test:
       timeout:
         minutes: 180