RFR: 8330523: Reduce runtime and improve efficiency of KeepAliveTest [v5]

[Christoph Langer]

> The test case sun/net/www/http/HttpClient/KeepAliveTest.java could be more effective.
> 
> It tests a matrix of HTTP client settings and server behavior, resulting in 160 individual test scenarios. Each is tested in an own freshly spawned JVM via the `@run main/othervm` directive. The need for new VMs is due to the fact that the behavior of the HTTP client is determined at VM initialization and can not be changed later on. However, for each distinct type of client settings, one VM can be reused. This would lead us from 160 JVM instantiations down to 16 which has a factor 10 influence on test runtime.
> 
> E.g. on my developer laptop runtime went down from ~100s to ~10s.
> 
> I also made additional cleanups/refactoring in the test.

Christoph Langer has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Merge branch 'master' into JDK-8330523
 - Review Feedback Daniel
 - Merge branch 'master' into JDK-8330523
 - Simplify the test further
 - Merge branch 'master' into JDK-8330523
 - Small further cleanup
 - JDK-8330523

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18817/files
  - new: https://git.openjdk.org/jdk/pull/18817/files/88572af1..b141813a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18817&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18817&range=03-04

  Stats: 9876 lines in 314 files changed: 6018 ins; 2754 del; 1104 mod
  Patch: https://git.openjdk.org/jdk/pull/18817.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18817/head:pull/18817

PR: https://git.openjdk.org/jdk/pull/18817