Add test for IDs for search entries of standalone comments

Exhibit a bug!

Signed-off-by: Paul-Elliot <peada@free.fr>

Author: panglesd

test/search/id_standalone_comments.t/main.ml

@@ -0,0 +1,11 @@
+let _ = 1
+
+(** standalone 1 *)
+
+module X = struct
+  let _ = 1
+
+  (** standalone 2 *)
+end
+
+(** standalone 3 *)

test/search/id_standalone_comments.t/run.t

@@ -0,0 +1,20 @@
+Compile the files
+
+  $ ocamlc -c main.ml -bin-annot -I .
+
+Compile and link the documentation
+
+  $ odoc compile -I . main.cmt
+  $ odoc link -I . main.odoc
+
+  $ odoc compile-index main.odocl
+
+Let's have a look at the links generated for standalone comments search entries:
+
+  $ cat index.json | jq -r '.[] | select(.kind.kind | contains("Doc")) | "\(.doc) -> \(.display.url)"'
+  standalone 1 -> Main/index.html
+  standalone 2 -> Main/X/index.html
+  standalone 3 -> Main/X/index.html
+
+There is a problem with "standalone 3": it should link to Main/index.html, not Main/X/index.html
+This is a bug!