Fix Socket.tcp cleanup after Thread#kill (#15131)

Socket.tcp launches ruby threads to resolve hostnames, and those threads
communicate through a queue implemented with `IO.pipe`. When the thread
that called `Socket.tcp` is killed, the resolver threads still try to
communicate through the pipe even though it may be closed. The method
`Socket.tcp_with_fast_fallback` tries to deal with this by killing the
threads in an ensure block, and then closing the pipe. However, calling
`Thread#kill` is not a blocking operation, it only sets a flag on the
thread telling it to raise during the next interrupt. The thread needs
to be joined to ensure it is terminated. The following script
demonstrates the issue:

```ruby
require "socket"

ts = []
5.times do
  ts << Thread.new do
    loop do
      1_000.times do |i|
        puts "#{i}"
        t = Thread.new do
          Socket.tcp("ruby-lang.org", 80)
        end
        sleep 0.001
        t.kill
      end
    end
  end
end
ts.each(&:join)
```

output:
```
/Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:1019:in 'IO#write': closed stream (IOError)
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:1019:in 'IO#putc'
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:1019:in 'block in Socket::HostnameResolutionResult#add'
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:1017:in 'Thread::Mutex#synchronize'
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:1017:in 'Socket::HostnameResolutionResult#add'
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:980:in 'Socket.resolve_hostname'
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:719:in 'block (2 levels) in Socket.tcp_with_fast_fallback'
/Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:1019:in 'IO#write': closed stream (IOError)
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:1019:in 'IO#putc'
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:1019:in 'block in Socket::HostnameResolutionResult#add'
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:1017:in 'Thread::Mutex#synchronize'
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:1017:in 'Socket::HostnameResolutionResult#add'
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:978:in 'Socket.resolve_hostname'
	from /Users/luke/workspace/ruby-dev/ruby-build-debug/.ext/common/socket.rb:719:in 'block (2 levels) in Socket.tcp_with_fast_fallback'
```

ext/socket/lib/socket.rb

@@ -917,7 +917,8 @@ class Socket < BasicSocket
     end
   ensure
     hostname_resolution_threads.each do |thread|
-      thread.exit
+      thread.kill
+      thread.join
     end
 
     hostname_resolution_result&.close