RFR: 8330523: Reduce runtime and improve efficiency of KeepAliveTest

[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 cases. Each scenario 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 type of client settings, the 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 some additional cleanups in the test.

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Commit messages:
 - JDK-8330523

Changes: https://git.openjdk.org/jdk/pull/18817/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18817&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330523
  Stats: 625 lines in 1 file changed: 73 ins; 366 del; 186 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