Add guards against `HttpHeaders::firstValueAsLong` failures. `H3MalformedResponseTest` is overhauled – migrated to JUnit, reinforced with exception type tests, etc. – but not all `firstValueAsLong` changes are verified with new additional tests. `test/jdk/java/net/httpclient/` tests still do pass.
------------- Commit messages: - Improve reuse - Improve wording in test - Improve wording in test - Guard `firstValueAsLong()` usages - Assert thrown exception types - Convert to JUnit - Add QPACK crash course Changes: https://git.openjdk.org/jdk/pull/28431/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28431&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8369595 Stats: 653 lines in 7 files changed: 234 ins; 128 del; 291 mod Patch: https://git.openjdk.org/jdk/pull/28431.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/28431/head:pull/28431 PR: https://git.openjdk.org/jdk/pull/28431
