CI - Check smoketests for changes (#4282)

# Description of Changes

`Cargo.lock` was getting stale in our smoketests modules, because we
don't check it anywhere. I added a check.

# API and ABI breaking changes

CI only.

# Expected complexity level and risk

1

# Testing

- [x] new CI step fails without the `Cargo.lock` changes
(<https://github.com/clockworklabs/SpacetimeDB/actions/runs/21960035403/job/63435284426?pr=4282>)
- [x] new CI step passes with the `Cargo.lock` changes

---------

Co-authored-by: Zeke Foppa <bfops@users.noreply.github.com>

Author: bfops

.github/workflows/ci.yml

@@ -164,6 +164,13 @@ jobs:
           }
           cargo ci smoketests -- --test-threads=1          
 
+      - name: Check for changes
+        run: |
+          tools/check-diff.sh crates/smoketests || {
+              echo 'Error: There is a diff in the smoketests directory.'
+              exit 1
+          }
+
   smoketests-python:
     needs: [lints]
     name: Smoketests (Python Legacy) (${{matrix.name}})