RFR: 8378344: Refactor test/jdk/java/net/httpclient/*.java TestNG tests to JUnit

[Daniel Fuchs]

TestNG tests directly located in test/jdk/java/net/httpclient/ are refactored to use JUnit.

This is a mostly mechanical change - though I fixed some compiker warnings along teh way.

Test only.

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Commit messages:
 - 8378344: Refactor test/jdk/java/net/httpclient/*.java TestNG tests to JUnit

Changes: https://git.openjdk.org/jdk/pull/29850/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29850&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378344
  Stats: 3201 lines in 79 files changed: 577 ins; 394 del; 2230 mod
  Patch: https://git.openjdk.org/jdk/pull/29850.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29850/head:pull/29850

PR: https://git.openjdk.org/jdk/pull/29850