RFR: 8297200: java/net/httpclient/SpecialHeadersTest.java failed once in AssertionError due to selector thread remaining alive

[Daniel Fuchs]

The java/net/httpclient/SpecialHeadersTest.java test has been observed failing once, due to a selector thread still running after all operations had terminated. This change slightly alter the test logic to check the shared client only at the end of the test when all methods have been executed, and focussed the checks performed by a test method to the only clients that this method is using. This may or may not prevent the observed issue to ever reproducing, but should at least provide more information on where the selector manager thread is at the time it is observed running, should the test fail again:
the logic of the HttpClient should have woken up the selector when the last operation finished, which should have ensured a prompt termination of the thread.

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Commit messages:
 - 8297200
 - 8297200

Changes: https://git.openjdk.org/jdk/pull/11294/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11294&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297200
  Stats: 18 lines in 2 files changed: 8 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/11294.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11294/head:pull/11294

PR: https://git.openjdk.org/jdk/pull/11294