test/net/smtp/test_smtp.rb: wait a moment before socket is closed

On Solaris, Socket.tcp seems to fail with EINVAL if the server closes
the connection immediately after accpeted. I think this is a bug of
Socket.tcp, but seems difficult to fix soon.

http://rubyci.s3.amazonaws.com/solaris11-sunc/ruby-master/log/20210429T100007Z.fail.html.gz
```
  1) Failure:
Net::TestSMTP#test_eof_error_backtrace [/export/home/chkbuild/chkbuild-sunc/tmp/build/20210429T100007Z/ruby/test/net/smtp/test_smtp.rb:193]:
[ruby-core:78550] [Bug #13018].
[EOFError] exception expected, not #<Net::ReadTimeout: Net::ReadTimeout>.
```

test/net/smtp/test_smtp.rb

@@ -187,6 +187,7 @@ def test_eof_error_backtrace
         sock = nil
         t = Thread.start do
           sock = accept(servers)
+          sleep 0.1
           sock.close
         end
         smtp = Net::SMTP.new("localhost", servers[0].local_address.ip_port)