Add test for hasSpecialChars

[elliotwutingfeng]

func hasSpecialChars(s string) bool {
	if strings.IndexByte(s, '"') >= 0 || strings.IndexByte(s, '\\') >= 0 {
		return true
	}
	for i := 0; i < len(s); i++ {
		if s[i] < 0x20 {
			return true
		}
	}
	return false
}

This PR modifies testArena to cover if s[i] < 0x20 in hasSpecialChars; improves overall test coverage slightly.