Run test-tidy when landing changes

Currently `test-tidy` only runs during the quick check. This can mean
that when a change is approved, the quick check can be skipped and a
change can land without `test-tidy` ever being run on it. This change
adds a `test-tidy` run to the main CI run to avoid this happening.

.github/workflows/main.yml

@@ -174,6 +174,8 @@ jobs:
         run: ./etc/ci/lockfile_changed.sh
       - name: Forbidden panic check
         run: ./etc/ci/check_no_panic.sh
+      - name: Tidy
+        run: python3 ./mach test-tidy --no-progress --all
       - name: Package binary
         run: tar -czf target.tar.gz target/release/servo resources
       - name: Archive binary