Add test exhibiting issue #944

Author: gpetiot

test/xref2/github_issue_944.t/foo.mli

@@ -0,0 +1 @@
+val f : Set.Make(String).t -> unit

test/xref2/github_issue_944.t/run.t

@@ -0,0 +1,10 @@
+A quick test to repro the issue found in #944
+
+  $ ocamlc -bin-annot -c foo.mli
+
+  $ odoc compile foo.cmti
+  $ odoc link foo.odoc
+  File "foo.odoc":
+  Warning: Failed to lookup type unresolvedroot(Stdlib).Set.Make(unresolvedroot(Stdlib).String).t Parent_module: Unresolved apply
+
+  $ odoc html-generate --indent -o html/ foo.odocl