Merge forbidden panic check into test-tidy

This cleans up the GitHub actions yaml a bit and ensures that developers
are running this check locally before submitting changes. In addition,
it allows adding tests for this check. Finally, this change fixes the
tidy tests by upgrading voluptuous for Python 3.10 as well as by
reverting an inadvertent change for NixOS compatibility on one of the
dummy testing files.

.github/workflows/main.yml

@@ -175,8 +175,6 @@ jobs:
         run: python3 ./mach build --release
       - name: Lockfile check
         run: ./etc/ci/lockfile_changed.sh
-      - name: Forbidden panic check
-        run: ./etc/ci/check_no_panic.sh
       - name: Package binary
         run: tar -czf target.tar.gz target/release/servo resources
       - name: Archive binary