RFR: 8292381: java/net/httpclient/SpecialHeadersTest.java fails with "ERROR: Shutting down connection: HTTP/2 client stopped" [v2]

[Daniel Fuchs]

> Please find here a change that improves SpecialHeadersTest. This test creates a large amount of ephemeral clients and has been observed running out of heap space in our CI once. This change updates the test to wait for the previous HttpClient to be eligible for garbage collection before it creates a new one. It also verifies that no outstanding operation are still running on the client by the time the client is released.

Daniel Fuchs has updated the pull request incrementally with one additional commit since the last revision:

  Integrated review feedback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9908/files
  - new: https://git.openjdk.org/jdk/pull/9908/files/3d951b08..cd588b71

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9908&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9908&range=00-01

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/9908.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9908/head:pull/9908

PR: https://git.openjdk.org/jdk/pull/9908