ocamltest: add the add_if_undefined function to environments

ocamltest/environments.ml

@@ -67,6 +67,9 @@ let is_variable_defined variable env =
 
 let add variable value env = VariableMap.add variable value env
 
+let add_if_undefined variable value env =
+  if VariableMap.mem variable env then env else add variable value env
+
 let append variable appened_value environment =
   let previous_value = safe_lookup variable environment in
   let new_value = previous_value ^ appened_value in

ocamltest/environments.mli

@@ -34,6 +34,7 @@ val lookup_as_bool : Variables.t -> t -> bool option
     [None] if not set. *)
 
 val add : Variables.t -> string -> t -> t
+val add_if_undefined : Variables.t -> string -> t -> t
 val add_bindings : (Variables.t * string) list -> t -> t
 
 val append : Variables.t -> string -> t -> t