RFR: 8320836: jtreg gtest runs should limit heap size

[Thomas Stuefe]

Trivial fix to limit Java heap size when running the gtests. Otherwise, we run with an unknown malloc load (since GC structures, preallocated, may be very large), and not all tests like that.

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Commit messages:
 - start

Changes: https://git.openjdk.org/jdk/pull/26454/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26454&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320836
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26454.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26454/head:pull/26454

PR: https://git.openjdk.org/jdk/pull/26454