[jdk17u-dev] RFR: 8303457: Introduce convenience test library APIs for creating test servers for tests in test/jdk/java/net/httpclient

[Goetz Lindenmaier]

I backport this for parity with 17.0.12-oracle.

test/jdk/java/net/httpclient/CancelRequestTest.java
test/jdk/java/net/httpclient/HeadTest.java
Resolved imports.

Omitted files:
test/jdk/java/net/httpclient/AsyncExecutorShutdown.java
test/jdk/java/net/httpclient/CancelStreamedBodyTest.java
test/jdk/java/net/httpclient/ExecutorShutdown.java
test/jdk/java/net/httpclient/ExpectContinueTest.java
test/jdk/java/net/httpclient/Response1xxTest.java
All these already had to be omitted in https://github.com/openjdk/jdk17u-dev/pull/2311,
they all came with new features or other changes that should
not be backported as prereq.

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Commit messages:
 - Backport 72de24e59a80a38ea4ea6a8a3f966f555987ac86

Changes: https://git.openjdk.org/jdk17u-dev/pull/2350/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2350&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303457
  Stats: 491 lines in 41 files changed: 165 ins; 143 del; 183 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/2350.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2350/head:pull/2350

PR: https://git.openjdk.org/jdk17u-dev/pull/2350