test: metadata in describe

Michael Vogt pointed out that testcases start failing when we describe
the new format. Let's add a test case and fix the describe to include
the metadata.

Metadata is freeform in the `Manifest` instance but it is stored on it
during loading (at which time its properties are validated) and returned
as-is on describe.

Signed-off-by: Simon de Vlieger <supakeen@redhat.com>

test/mod/test_fmt_v2.py

@@ -12,6 +12,14 @@ import osbuild.meta
 
 BASIC_PIPELINE = {
     "version": "2",
+    "metadata": {
+        "generators": [
+            {
+                "name": "handcrafted",
+                "version": "NaN",
+            },
+        ],
+    },
     "sources": {
         "org.osbuild.curl": {
             "items": {