OCaml 5: misc testsuite fixes (#254)

* Revert `lib-hashtbl/htbl.ml` to tip-jst, plus a few tweaks

* Comment out parts of test that rely on as-of-yet-unimplemented `String.hash`

* Delete `testsuite/tests/misc/ephetest2.ml` in favor of the equivalent test `testsuite/tests/weak-ephe-final/ephetest2.ml`

* Build dumpobj in tests