RFR: 7903428: jcstress: Adjust max memory per test

[Aleksey Shipilev]

There is an evidence that some tests fragment the heap significantly, without any recourse -- for example with humongous allocations in the large array tests. The current estimate of "take half of the heap for the test" is cutting it dangerously close. G1 routinely fails some of the init.arrays.large tests because of this.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jcstress/pull/136/files
 Webrev: https://webrevs.openjdk.org/?repo=jcstress&pr=136&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903428
  Stats: 9 lines in 2 files changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jcstress/pull/136.diff
  Fetch: git fetch https://git.openjdk.org/jcstress pull/136/head:pull/136

PR: https://git.openjdk.org/jcstress/pull/136