RFR: 8274517: java/util/DoubleStreamSums/CompensatedSums.java fails with expected [true] but found [false]

[Raffaello Giulietti]

Please review these small changes to address intermittent failures, as of JDK-8274517.

- Usage of jdk.test.lib.RandomFactory for reproducible random generation.
- Slightly less restrictive assertion about badParallelStreamError on L94 (former L88).
- Verbatim copy of computeFinalSum() from j.u.s.Collectors 18.

While these changes do not necessarily guarantee absence of intermittent failures, the usage of jdk.test.lib.RandomFactory should at least help to pin down specific double sequences that do not pass the test.

There is still an inherent variability due to the use of parallel streams, though. As double addition is not perfectly associative, even a fully known sequence of doubles may lead to slightly different results with parallelization.

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Commit messages:
 - 8274517: java/util/DoubleStreamSums/CompensatedSums.java fails with expected [true] but found [false]

Changes: https://git.openjdk.java.net/jdk/pull/8290/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8290&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274517
  Stats: 13 lines in 1 file changed: 7 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8290.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8290/head:pull/8290

PR: https://git.openjdk.java.net/jdk/pull/8290