ci: Require go.mod to be tidy

This helps us prevent accidentally having an inconsistent go.mod file
after changes to dependencies.

.github/workflows/test.yml

@@ -41,6 +41,9 @@ jobs:
       - uses: actions/setup-go@v2
         with:
           go-version: ${{ matrix.go }}
+      - name: Check go.mod Tidiness
+        run: go mod tidy && git diff --exit-code
+        if: ${{ matrix.go == '1.16' }}
       - name: Build
         run: go build ./...
       - name: Vet