RFR: 8378164: test/jdk/java/net/httpclient/http3/*.java: convert tests that use ITestContext to JUnit

[Daniel Fuchs]

Similar to https://github.com/openjdk/jdk/pull/29786 - this time with HTTP/3 tests.

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Commit messages:
 - 8378164: test/jdk/java/net/httpclient/http3/*.java: convert tests that use ITestContext to JUnit

Changes: https://git.openjdk.org/jdk/pull/29824/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29824&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378164
  Stats: 325 lines in 4 files changed: 120 ins; 71 del; 134 mod
  Patch: https://git.openjdk.org/jdk/pull/29824.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29824/head:pull/29824

PR: https://git.openjdk.org/jdk/pull/29824