Add test for boolean not

src/contracts/boolean_operators.ligo

@@ -11,3 +11,6 @@ function and_true (const b : bool) : bool is
 
 function and_false (const b : bool) : bool is
   begin skip end with b and False
+
+function not_bool (const b: bool) : bool is
+  begin skip end with not b

src/test/integration_tests.ml

@@ -127,6 +127,7 @@ let bool_expression () : unit result =
       ("or_false", fun b -> b || false) ;
       ("and_true", fun b -> b && true) ;
       ("and_false", fun b -> b && false) ;
+      ("not_bool", fun b -> not b) ;
     ] in
   ok ()