[jdk25u-dev] RFR: 8318662: Refactor some jdk/java/net/httpclient/http2 tests to JUnit

[Goetz Lindenmaier]

I backport this for parity with 25.0.3-oracle.

Omitted one edit in ContinuationFrameTest.java in code added by [JDK-8349910](https://bugs.openjdk.org/browse/JDK-8349910)

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Commit messages:
 - backport 7fe71a78137991d26553b6d5e0d4d74900f01ba3

Changes: https://git.openjdk.org/jdk25u-dev/pull/327/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u-dev&pr=327&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318662
  Stats: 132 lines in 5 files changed: 5 ins; 30 del; 97 mod
  Patch: https://git.openjdk.org/jdk25u-dev/pull/327.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u-dev.git pull/327/head:pull/327

PR: https://git.openjdk.org/jdk25u-dev/pull/327