ipv6: fixed ipv6 address check in @ipv6_describe

Since in new versions of NM the [NM property description] field in description of ipv6 addresses in nmcli connection editor got changed -

from: "Array of IP addresses."

to: "A list of IPv6 addresses and their prefix length. Multiple addresses can be separated by comma. For example "2001:db8:85a3::8a2e:370:7334/64, 2001:db8:85a3::5/64". The addresses are listed in increasing priority, meaning the last address will be the primary address."

@ipv6_describe test had to be slightly modified to match both texts.

Other chanages include adding extra whitespace character to match the common indentation levels present in test suite.