run tests consecutively to avoid constraint violation

.github/workflows/tests.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Build
         run: cargo build --verbose
       - name: Run tests
-        run: cargo test --verbose
+        run: cargo test --verbose -- --test-threads=1
         env:
           DATABASE_URL: "postgres://postgres:postgres@localhost:5432/postgres"
           PORT: 8080