RFR: 8321124: java/util/stream/GatherersTest.java times out [v5]

[Viktor Klang]

> Splits GatherersTest out to distinct files per built-in gatherer
> 
> Fixes a test ordering issue for mapConcurrent which placed GatherersTest on the ProblemList.
> 
> Adding increased maxOutputSize for Gatherer-related tests to improve debuggability.
> 
> Lowering the composition threshold of GathererTest.testMassivelyComposedGatherers to 256 to avoid SOE on low-specc:ed machines.

Viktor Klang has updated the pull request incrementally with one additional commit since the last revision:

  Addressing review feedback: improving test coverage and debuggability

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16928/files
  - new: https://git.openjdk.org/jdk/pull/16928/files/fd426ae7..663a7b64

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16928&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16928&range=03-04

  Stats: 76 lines in 1 file changed: 49 ins; 9 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/16928.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16928/head:pull/16928

PR: https://git.openjdk.org/jdk/pull/16928