Handle the interface name in IPv6 address correctly

Motivation:

NetUtil.isValidIpV6Address() handles the interface name in IPv6 address
incorrectly. For example, it returns false for the following addresses:

- ::1%lo
- ::1%_%_in_name_

Modifications:

- Strip the square brackets before validation for simplicity
- Strip the part after the percent sign completely before validation for
  simplicity
- Simplify and reformat NetUtilTest

Result:

- The interface names in IPv6 addresses are handled correctly.
- NetUtilTest is cleaner