RFR: 8263464: NMT: assert in gtest os.release_multi_mappings_vm [v3]

[Gerard Ziemski]

> This fix allows NMT to account for released memory that was allocated in chunks, as long as the final pointer and size refer to total contiguous regions of the requested size.
> 
> Tested via `Mach5 tier1,tier2,tier3` and locally via `gtest:NMT*:os*`

Gerard Ziemski has updated the pull request incrementally with one additional commit since the last revision:

  add support for more cases of partial regions, make test more flexible, add more tests

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13813/files
  - new: https://git.openjdk.org/jdk/pull/13813/files/7d6a6c0c..c356ad0e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13813&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13813&range=01-02

  Stats: 122 lines in 2 files changed: 62 ins; 13 del; 47 mod
  Patch: https://git.openjdk.org/jdk/pull/13813.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13813/head:pull/13813

PR: https://git.openjdk.org/jdk/pull/13813