Correctly format test URI's that contain an IPv6 address retrieved from the host

Several tests that retrieve an IP address from the host system, use that in the construction of a URL / URI. If the IP address is an IPv6 address, then it may need to be enclosed in "[" and "]" characters.

Follow-on to JDK-8220575, but with a slightly different pattern, since the tests use getLocalHost, rather than getLoopbackAddress or 127.0.0.1, etc.

Two such tests are:
  test/jdk/sun/net/www/protocol/http/HttpOnly.java
  test/jdk/sun/net/www/protocol/http/NoCache.java

These test don't need to use getLocalHost, they could use the loopback.