RFR: 8330523: Reduce runtime and improve efficiency of KeepAliveTest [v7]

[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 incrementally with one additional commit since the last revision:

  Write server thread code more concisely

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18817/files
  - new: https://git.openjdk.org/jdk/pull/18817/files/6dc20dc5..12702b8a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18817&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18817&range=05-06

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 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