RFR: 8378344: Refactor test/jdk/java/net/httpclient/*.java TestNG tests to JUnit [v2]

[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.

Daniel Fuchs has updated the pull request incrementally with three additional commits since the last revision:

 - Update test/jdk/java/net/httpclient/HttpVersionsTest.java
   
   Co-authored-by: Volkan Yazıcı <volkan.yazici at oracle.com>
 - Update test/jdk/java/net/httpclient/HttpVersionsTest.java
   
   Co-authored-by: Volkan Yazıcı <volkan.yazici at oracle.com>
 - Update test/jdk/java/net/httpclient/BufferingSubscriberErrorCompleteTest.java
   
   Co-authored-by: Volkan Yazıcı <volkan.yazici at oracle.com>

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/29850/files
  - new: https://git.openjdk.org/jdk/pull/29850/files/d08236d5..13f03645

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29850&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29850&range=00-01

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 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