Fix tests.py paths and use pre-commit to guard against lost tests (#689)

- In pre-commit-config, add a run of `--update_list`. I'm doing this because I noticed (on running tests.py) that we'd lost track of a few tests.
- In tests.py, the paths affect bison output where the path to the file is mentioned. This shift should make `--update_goldens` more reliable.

.pre-commit-config.yaml

@@ -86,6 +86,14 @@ repos:
         exclude: '^website/jekyll/Gemfile.lock$'
   - repo: local
     hooks:
+      - id: executable-semantics-tests
+        name: Update list of executable_semantics tests
+        description: Updates executable_semantics/test_list.bzl
+        entry: executable_semantics/tests.py
+        language: python
+        files: ^executable_semantics/(test_list.bzl|testdata/.*\.carbon)$
+        pass_filenames: false
+        args: ['--update_list', '--use_git_ls_files']
       - id: proposal-list
         name: Update list of proposals
         description: Updates the list of proposals in proposals/README.md