build: add h2-eio and eio_main as test deps for grpc-eio test_concurrent_handler.ml imports H2_eio and Eio_main, which live in the h2-eio and eio_main opam packages respectively. Without declaring them as {with-test} dependencies, `opam install ./grpc-eio.opam --with-test` fails in a clean switch. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>