feat(tests): cmd executeCommand captures stderr & stdout (#769)

test only refactor that will be used by https://code.forgejo.org/forgejo/runner/pulls/757

<!--start release-notes-assistant-->
<!--URL:https://code.forgejo.org/forgejo/runner-->
- features
  - [PR](https://code.forgejo.org/forgejo/runner/pulls/769): <!--number 769 --><!--line 0 --><!--description ZmVhdCh0ZXN0cyk6IGNtZCBleGVjdXRlQ29tbWFuZCBjYXB0dXJlcyBzdGRlcnIgJiBzdGRvdXQ=-->feat(tests): cmd executeCommand captures stderr & stdout<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/769
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

internal/app/cmd/main_test.go

@@ -0,0 +1,52 @@
+// Copyright 2025 The Forgejo Authors
+// SPDX-License-Identifier: MIT
+
+package cmd
+
+import (
+	"bytes"
+	"context"
+	"io"
+	"os"
+	"testing"
+
+	"code.forgejo.org/forgejo/runner/testutils"
+
+	"github.com/spf13/cobra"
+	"github.com/stretchr/testify/require"
+)
+
+// Capture what's being written into a standard file descriptor.
+func captureOutput(t *testing.T, stdFD *os.File) (finish func() (output string)) {
+	t.Helper()
+
+	r, w, err := os.Pipe()
+	require.NoError(t, err)
+	resetStdout := testutils.MockVariable(stdFD, *w)
+
+	return func() (output string) {
+		w.Close()
+		resetStdout()
+
+		out, err := io.ReadAll(r)
+		require.NoError(t, err)
+		return string(out)
+	}
+}
+
+func executeCommand(ctx context.Context, t *testing.T, cmd *cobra.Command, args ...string) (cmdOut, stdOut, stdErr string, err error) {
+	t.Helper()
+	finishStdout := captureOutput(t, os.Stdout)
+	finishStderr := captureOutput(t, os.Stderr)
+	buf := new(bytes.Buffer)
+	cmd.SetOut(buf)
+	cmd.SetErr(buf)
+	cmd.SetArgs(args)
+
+	err = cmd.ExecuteContext(ctx)
+
+	cmdOut = buf.String()
+	stdOut = finishStdout()
+	stdErr = finishStderr()
+	return cmdOut, stdOut, stdErr, err
+}