Test-cases for DNS entry removal bugs

Here are some test-cases for bugs https://sourceforge.net/p/curl/bugs/1303/ and https://sourceforge.net/p/curl/bugs/1327/. It checks the removal os DNS entries after DNS_CACHE_TIMEOUT elapses, both with and without a dead connection detection in the beginning of the second request. The tests show, however, that commit 030a2b8cb8c4f1d03af6f6e6a3aa3a5a1db5f5a7 is superfluous: 31860ab8c83b35b21f331c85e4535b08b13dc8f9 alone fixes both cases.