RFR: 8307626: java/net/httpclient/FlowAdapter* tests should close the HttpClient instances

[Daniel Fuchs]

java/net/httpclient/FlowAdapterPublisherTest.java and java/net/httpclient/FlowAdapterSubscriberTest.java create one HttpClient instance per test method. The test should use try-with-resources to close the client instances.

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Commit messages:
 - 8307626

Changes: https://git.openjdk.org/jdk/pull/13909/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13909&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307626
  Stats: 481 lines in 2 files changed: 88 ins; 59 del; 334 mod
  Patch: https://git.openjdk.org/jdk/pull/13909.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13909/head:pull/13909

PR: https://git.openjdk.org/jdk/pull/13909