chore: minor tweaks to `Sym.simp` test and benchmark

This PR applies two minor tweaks:
- `tests/bench/sym/simp_1.lean`: share-common the proof term before
  counting objects in `getProofSize`, so the reported size reflects
  the shared representation.
- `tests/elab/sym_simp_3.lean`: use `>>` instead of `.andThen` when
  composing `Sym.Simp` methods.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
2 files changed