Fix timeout option for Ping::External#ping6 on macOS

[taketo1113]

This Pull Request fixes an issue on macOS where Net::Ping::External#ping6 returns false even for reachable targets.

Steps to reproduce

• macOS: 15.3
• ruby: 3.4.1
• net-ping: 2.0.8

require 'net/ping'

pe = Net::Ping::External.new("2001:4860:4860::8888") # dns.google
pe.ping6
=> false

pe.exception
=> "ping6: nodename nor servname provided, or not known"

The ping6 command to the same target(2001:4860:4860::8888) succeeds.

$ ping6 -c 1 2001:4860:4860::8888
PING6(56=40+8+8 bytes) 2001:268:c281:1ade:c461:6f59:7200:5235 --> 2001:4860:4860::8888
16 bytes from 2001:4860:4860::8888, icmp_seq=0 hlim=113 time=38.147 ms

--- 2001:4860:4860::8888 ping6 statistics ---
1 packets transmitted, 1 packets received, 0.0% packet loss
round-trip min/avg/max/std-dev = 38.147/38.147/38.147/0.000 ms

Expected behavior

Ensure that Net::Ping::External#ping6 succeeds and returns true on macOS.

pe = Net::Ping::External.new("2001:4860:4860::8888") # dns.google
pe.ping6
=> true

Actual behavior

Net::Ping::External#ping6 return false even for reachable targets.

pe = Net::Ping::External.new("2001:4860:4860::8888") # dns.google
pe.ping6
=> false

pe.exception
=> "ping6: nodename nor servname provided, or not known"

On macOS, Net::Ping::External#ping6 executes the following command (with -t option), and the same error is returned.

$ ping6 -c 1 -t 5 2001:4860:4860::8888
ping6: nodename nor servname provided, or not known

Details

It fixes the timeout option for Net::Ping::External#ping6 on macOS.
macOS's ping6 does not have the -t option to specify a timeout, so the -i option is used instead.

$ man ping6
...
     -i wait
             Wait wait seconds between sending each packet.  The default is to
             wait for one second between each packet.  The wait time may be
             fractional, but only the super-user may specify values less than
             0.002 second.  This option is incompatible with the -f option.

For reference, I have included the contents of the ping6 man page from macOS 15.3 here:
https://gist.github.com/taketo1113/4cdae90f7344b121ce051562def4ba21