Fix flaky test case for synchrony driver

[supercaracal]

This PR will resolve #943.

redis-rb/lib/redis/connection/synchrony.rb

Lines 92 to 94 in cf44d71

	conn = EventMachine.connect(config[:host], config[:port], RedisClient) do |c|
	c.pending_connect_timeout = [config[:connect_timeout], 0.1].max
	end

• Doesn't connect via localhost eventmachine/eventmachine#885
• It seems that the implementation using only socket(2) is not enough.
• EventMachine::DNS::Resolver

on GitHub Actions

$ cat /etc/hosts
127.0.0.1 localhost
---snip---

$ cat /etc/resolv.conf
---snip---
nameserver 127.0.0.53
options edns0
search equ4xg1rbhmubnhv1dj5iatrre.bx.internal.cloudapp.net

$ cat /etc/nsswitch.conf
---snip---
hosts:          files dns
---snip---

Ref: #915