Fix file creation for output options (#875)

test/cli/trait_assertions_test.go

@@ -10,10 +10,26 @@ import (
 	"testing"
 
 	"github.com/acarl005/stripansi"
+	"github.com/stretchr/testify/require"
 )
 
 type traitAssertion func(tb testing.TB, stdout, stderr string, rc int)
 
+func assertFileOutput(tb testing.TB, path string, assertions ...traitAssertion) traitAssertion {
+	tb.Helper()
+
+	return func(tb testing.TB, _, stderr string, rc int) {
+		content, err := os.ReadFile(path)
+		require.NoError(tb, err)
+		contentStr := string(content)
+
+		for _, assertion := range assertions {
+			// treat the file content as stdout
+			assertion(tb, contentStr, stderr, rc)
+		}
+	}
+}
+
 func assertJsonReport(tb testing.TB, stdout, _ string, _ int) {
 	tb.Helper()
 	var data interface{}