Add missing sourcemap flags to test

Author: OlivierNicole

compiler/tests-sourcemap/dune

@@ -1,6 +1,8 @@
 (env
  (_
-  (js_of_ocaml
+ (js_of_ocaml
+   (flags (--source-map))
+   (link_flags (--source-map --resolve-sourcemap-url true))
    (compilation_mode separate))))
 
 (executable