RFR: 8270553: Tests should not use (real, in-use, routable) 1.1.1.1 as dummy IP value

[Jonathan Dowland]

The tests `test/jdk/java/net/HttpURLConnection/HttpURLConWithProxy.java` uses the IP address "1.1.1.1" as a value. I think at the time the address was picked, the assumption was the address was not valid / not routable. Since April 2018 the address is part of CloudFlare's "Free" DNS product: <https://en.wikipedia.org/wiki/1.1.1.1>. (this test was originally written in 2016, before the service was launched)

I've verified using local packet captures that running the test does result in IP traffic being sent to 1.1.1.1. (Several other tests in JDK use 1.1.1.1 as a placeholder IP. I've checked them all and none of the others connect out to the IP like this one)
 
This PR substitutes that IP address value (and two others) for ones from a reserved IP range (240.0.0.0/4 according to RFC 6761) which will not result in runners of the test suit inadvertently sending IP packets to the CloudFlare service. 

This could be invalidated again if that address range is allocated at some point in the future. A more future-proof fix would be to bind to random ports on localhost for each dummy proxy (as done for the target HTTP server in the test already). I can do that if preferred.

<https://bugs.openjdk.java.net/browse/JDK-8270553>

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Commit messages:
 - 8270553: Tests should not use (real, in-use, routable) 1.1.1.1 as dummy IP value

Changes: https://git.openjdk.java.net/jdk/pull/4806/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4806&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270553
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4806.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4806/head:pull/4806

PR: https://git.openjdk.java.net/jdk/pull/4806