Add schema-related CI test coverage

Author: ZackPierce

.github/workflows/tests.yaml

@@ -33,6 +33,12 @@ jobs:
           command: test
           args: --no-default-features
 
+      - name: Test (schema features subset)
+        uses: actions-rs/cargo@v1
+        with:
+          command: test
+          args: --features "std,schemars"
+
       - name: Test (all features)
         uses: actions-rs/cargo@v1
         with: