ci-test: Fix syntax/formatting check so it actually emits an exit code

The old format didn't emit any exit codes and thus did not cause CI to
fail:

    $ diff -u <(echo -n) <(gofmt -d -s .)
    make.go:513:46: missing ',' before newline in composite literal
    $ echo $?
    0

The old code would have an failure only if `gofmt` ran successfully AND
outputted changes. With the new structure both `gofmt` and `git` needs
to pass, thus capturing exit codes and error of any kind of failures.

Author: ottok

.github/workflows/ci-test.yml

@@ -23,7 +23,7 @@ jobs:
           go-version: ${{ matrix.go-version }}
           check-latest: true
 
-      - run: diff -u <(echo -n) <(gofmt -d -s .)
+      - run: gofmt -w -s . && git diff --exit-code --color
 
       - run: go install -v ./...