tests: Add failing test with repository includes

Add a new test to test_commands.py which:
 * Has an incomplete definition of externalrepo
 * Defines a local repository, subrepo
 * Includes a file from subrepo
 * The included file completes the definition of externalrepo
 * It also attempts to include a file from externalrepo

This test previously failed with the error message "No such file or
directory: '.../test_commands/tests/test_layers/test.yml'"

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

tests/test_repo_includes/subrepo/test.yml

@@ -0,0 +1,10 @@
+header:
+  version: 11
+  includes:
+    - repo: externalrepo
+      file: tests/test_layers/test.yml
+
+repos:
+  externalrepo:
+    url: https://github.com/siemens/kas.git
+    path: externalrepo