.github/workflows: check formatting

Check gofmt formatting as part of GitHub actions tests.

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

.github/workflows/test.yml

@@ -35,5 +35,9 @@ jobs:
         go mod tidy
         git diff --exit-code
 
+    - name: Check formatting
+      if: matrix.go-version == '1.17'
+      run: diff -u <(echo -n) <(gofmt -d .)
+
     - name: Run unit tests
       run: go test -v ./...