[lworld] RFR: 8336672: [lworld] Tests of spliterators and streams fail when value objects are used with WeakHashmap

[Roger Riggs]

Update stream and spliterator tests of WeakHashmap that use Integer, a value class, as test data. 

The Integers are replaced with Strings as test data in most tests. 
In others, the intersection of Collections with value classes (Integer) are disabled when running with --enable-preview.

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Commit messages:
 - 8336672-stream-weakhashmap

Changes: https://git.openjdk.org/valhalla/pull/1178/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1178&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336672
  Stats: 109 lines in 6 files changed: 19 ins; 4 del; 86 mod
  Patch: https://git.openjdk.org/valhalla/pull/1178.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1178/head:pull/1178

PR: https://git.openjdk.org/valhalla/pull/1178