Bug 24739: Perform IPv6 tests only if Net::Netmask is correct version

Bug 24739: Perform IPv6 tests only if Net::Netmask is correct version

Test plan:
0. Apply patch
1. Set up Koha on stretch (e.g. koha/koha-testing:master)
2. perl t/Koha/Middlware/RealIP.t
3. Observe the following:
Subtest: IPv6 support
    ok 1 - Warn on IPv6 koha_trusted_proxies
    ok 2 - Unable to parse IPv6 address for trusted proxy, so ignore the X-Forwarded-For header
    1..2
ok 13 - IPv6 support

4. Set up Koha on buster (e.g. koha/koha-testing:master-buster)
5. perl t/Koha/Middlware/RealIP.t
6. Observe the following:
Subtest: IPv6 support
    ok 1 - Trust proxy (2001:db8:1234:5678:abcd:1234:abcd:1234) using IPv6 CIDR notation, so use the X-Forwarded-For header for the remote address
    1..1
ok 13 - IPv6 support

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>