stages/cloud-init: test datasource_list dump format

Add a simple unit test, which ensures that the `datasource_list` key
is dumped in the configuration file on a single line if defined in
the stage options.

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

stages/test/test_cloud-init.py

@@ -0,0 +1,29 @@
+#!/usr/bin/python3
+
+import os.path
+
+from osbuild.testutil.imports import import_module_from_path
+
+
+# Test that the 'datasource_list' option is dumped as a single line
+# in the configuration file.
+def test_cloud_init_datasource_list_single_line(tmp_path):
+    treedir = tmp_path / "tree"
+    confpath = treedir / "etc/cloud/cloud.cfg.d/datasource_list.cfg"
+    confpath.parent.mkdir(parents=True, exist_ok=True)
+
+    stage_path = os.path.join(os.path.dirname(__file__), "../org.osbuild.cloud-init")
+    stage = import_module_from_path("stage", stage_path)
+
+    options = {
+        "filename": confpath.name,
+        "config": {
+            "datasource_list": [
+                "Azure",
+            ]
+        }
+    }
+
+    stage.main(treedir, options)
+    assert os.path.exists(confpath)
+    assert confpath.read_text() == "datasource_list: [Azure]\n"