func TestExitCodeFilter(t *testing.T) {
	// Write text to the filter one character at a time.
	var out strings.Builder
	f, exitStr := newExitCodeFilter(&out)
	// Embed a "fake" exit code in the middle to check that we don't get caught on it.
	pre := "abc" + exitStr + "123def"
	text := pre + exitStr + `1`
	for i := 0; i < len(text); i++ {
		_, err := f.Write([]byte{text[i]})
		if err != nil {
			t.Fatal(err)
		}
	}