conf: fix edge case in TOML config validation

rust2rpm/conf.py

@@ -459,6 +459,10 @@ class TomlConf:
                     if len(run) != 1:
                         raise ConfError(f"Invalid set of tests to run: {run!r}")
 
+        # the "default" feature is always implicitly defined
+        if "default" not in features:
+            features.add("default")
+
         if features_list := toml.get("features"):
             if enable_list := features_list.get("enable"):
                 for enabled in enable_list: