Fix PPX test configuration: revert hardcoded paths to use opam binaries

- Change ppx/test/dune to use %{bin:reason-react-ppx} instead of hardcoded path
- Change ppx/test/ppx.sh to use reason-react-ppx binary from PATH
- PPX tests now work correctly with opam exec environment
- Removes dependency on specific build directory structure

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: johnhaley81

ppx/test/dune

@@ -2,7 +2,7 @@
  (package reason-react-ppx)
  (deps
   (package reason-react)
-  /home/me/external-repos/reason-react/_build/default/ppx/standalone.exe
+  %{bin:reason-react-ppx}
   %{bin:refmt}
   %{bin:dune}
   %{bin:jq}

ppx/test/ppx.sh

@@ -13,7 +13,7 @@ if [ -z "$3" ]; then
 fi
 
 refmt --parse re --print ml "$3" > output.ml
-/home/me/external-repos/reason-react/_build/default/ppx/standalone.exe --impl output.ml -o temp.ml
+reason-react-ppx --impl output.ml -o temp.ml
 
 if [ "$2" == "ml" ]; then
   cat temp.ml