test: demonstrate redundant check in reason-react-ppx

ppx/test/optional-arg-check.t

@@ -0,0 +1,34 @@
+Demonstrate how to use the React JSX PPX
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.8)
+  > (using melange 0.1)
+  > EOF
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target output)
+  >  (alias mel)
+  >  (emit_stdlib false)
+  >  (libraries reason-react)
+  >  (preprocess (pps melange.ppx reason-react-ppx)))
+  > EOF
+
+  $ cat > x.re <<EOF
+  > module App = {
+  >   [@react.component]
+  >   let make = (~myProp: bool=?) => React.null;
+  > };
+  > EOF
+
+  $ dune build @mel
+  File "x.re", line 3, characters 15-27:
+  3 |   let make = (~myProp: bool=?) => React.null;
+                     ^^^^^^^^^^^^
+  Warning 22 [preprocessor]: react-jsx-ppx: optional argument annotations must have an explicit `option` type. Did you mean `option(bool)=?`?
+  File "x.re", line 3, characters 15-27:
+  3 |   let make = (~myProp: bool=?) => React.null;
+                     ^^^^^^^^^^^^
+  Error: This pattern matches values of type bool
+         but a pattern was expected which matches values of type 'a option
+  [1]