Run go mod tidy as part of GitHub actions

Make sure go.mod and go.sum match the current source in the module.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

.github/workflows/test.yml

@@ -29,5 +29,11 @@ jobs:
       if: matrix.go-version == '1.16' && matrix.platform == 'ubuntu-latest'
       run: diff -u <(echo -n) <(go fmt $(go list ./...))
 
+    - name: Check Go modules
+      if: matrix.go-version == '1.16'
+      run: |
+        go mod tidy
+        git diff --exit-code
+
     - name: Run unit tests
       run: go test -v ./...