tests: add comments to expected failure cases (#2608)

Signed-off-by: Mike Ralphson <mike.ralphson@gmail.com>

Author: MikeRalphson

tests/v3.1/fail/no_containers.yaml

@@ -1,4 +1,7 @@
-iopenapi: 3.1.0
+openapi: 3.1.0
+
+# this example should fail as there are no paths, components or webhooks containers (at least one of which must be present)
+
 info:
   title: API
   version: 1.0.0

tests/v3.1/fail/server_enum_empty.yaml

@@ -1,4 +1,7 @@
 openapi: 3.1.0
+
+# this example should fail as the server variable enum is empty, and so does not contain the default value
+
 info:
   title: API
   version: 1.0.0

tests/v3.1/fail/servers.yaml

@@ -1,4 +1,7 @@
 openapi: 3.1.0
+
+# this example should fail, as servers must be an array, not an object
+
 info:
   title: API
   version: 1.0.0

tests/v3.1/fail/unknown_container.yaml

@@ -1,4 +1,7 @@
 openapi: 3.1.0
+
+# this example should fail as overlays is not a valid top-level object/keyword
+
 info:
   title: API
   version: 1.0.0