Add a test illustrating issue #1913

Author: voodoos

tests/test-dirs/short-paths/double-trouble.t

@@ -0,0 +1,14 @@
+Found in issue #1913
+
+  $ cat >test.ml <<EOF
+  > type _ plus = | Zero : 'm plus |  Suc : 'm plus -> 'm plus
+  > type _ has_plus = Plus : 'm plus -> unit has_plus;;
+  > let (Plus (type mn3) (ed : mn3 plus) ) = Plus (Suc Zero) in ed + 2
+  > EOF
+
+  $ $MERLIN single errors -short-paths -filename test.ml < test.ml
+  {
+    "class": "failure",
+    "value": "Graph.add: type already defined: mn3/281[4]",
+    "notifications": []
+  }