RFR: JDK-8271242: add arena regression tests

[Thomas Stuefe]

Hi,

May I have reviews for these new regression tests for hotspot arenas, please? Arena coding is fragile, and recently we have changed it and plan to change it some more. But we have no tests, so regression tests should exist.

This is a breakout from https://github.com/openjdk/jdk/pull/4784. In which I rewrote arena alignment handling and added a lot of tests. The arena changes were rejected as too complex, which is okay, but most of the tests are still good and I hope not controversial. Therefore I would like the tests to the gtest suite.

This patch adds test, but does *not* change the Arenas themselves. Instead it tests current behavior, taking it as a baseline. Where the current behavior is inconsistent (eg. when dealing with 0-sized allocations), it nevertheless tests the inconsistent behavior, since callers may rely on it.

This patch also adds a new jtreg gtest wrapper which tests the arena-releated part of the tests with +UseMallocOnly. As long as we support that mode, we should test it.

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Tests:
- manual on Linux x64 and x86 both, with and without UseMallocOnly.
- GHAs
- nightlies at SAP are scheduled.

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Note: this needs JDK-8270308, which is not yet pushed but under review here: https://github.com/openjdk/jdk/pull/4835.

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Commit messages:
 - start
 - JDK-8270308-Amalloc-aligns-size-but-not-return-value

Changes: https://git.openjdk.java.net/jdk/pull/4898/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4898&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271242
  Stats: 442 lines in 4 files changed: 441 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4898.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4898/head:pull/4898

PR: https://git.openjdk.java.net/jdk/pull/4898