RFR: 8378595: Refactor miscellaneous tests under test/jdk/java/net/httpclient from TestNG to JUnit

[Daniel Fuchs]

A small changeset to convert TestNG tests under `test/jdk/java/net/httpclient/security`, `test/jdk/java/net/httpclient/offline`, and `test/jdk/java/net/httpclient/PathSubscriber` to JUnit.

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Commit messages:
 - 8378595: Refactor miscellaneous tests under test/jdk/java/net/httpclient from TestNG to JUnit

Changes: https://git.openjdk.org/jdk/pull/29900/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29900&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378595
  Stats: 174 lines in 7 files changed: 41 ins; 22 del; 111 mod
  Patch: https://git.openjdk.org/jdk/pull/29900.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29900/head:pull/29900

PR: https://git.openjdk.org/jdk/pull/29900