RFR: 8305900: Use loopback IP addresses in security policy files of httpclient tests

[Jaikiran Pai]

Can I please get a review of this test-only changes to the `java/net/httpclient/` tests? This addresses https://bugs.openjdk.org/browse/JDK-8305900.

As noted in the JBS issue, the changes here update the security policy files and the test library to use IP address of the loopback address instead of using "localhost".

Multiple runs of tests have been run with this change both with IPv4 and IPv6 and I haven't observed any failures or regressions with these changes.

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Commit messages:
 - 8305900: Use loopback IP addresses in security policy files of httpclient tests

Changes: https://git.openjdk.org/jdk/pull/13444/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13444&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305900
  Stats: 421 lines in 33 files changed: 187 ins; 30 del; 204 mod
  Patch: https://git.openjdk.org/jdk/pull/13444.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13444/head:pull/13444

PR: https://git.openjdk.org/jdk/pull/13444