Update a few java/net tests to use the loopback address instead of the host address

The following networking tests have been seen to fail from a timeout, when the host
is not accessible. These tests have nothing whatsoever to do with testing of network
connections, they simply want to test the HTTP client implementation. This issue
proposes to change the tests from using the host's name/address to using the
loopback, to make them more robust.

test/java/net/CookieHandler/CookieManagerTest.java
test/java/net/HttpURLConnection/UnmodifiableMaps.java
test/java/net/URLConnection/HandleContentTypeWithAttrs.java