RFR: 8349771: Replace usages of -mx and -ms in some monitor tests

[SendaoYan]

Hi all,
This PR updates two runtime/Monitor tests to replace the usage of the outdated `-mx` and `-ms` java launcher options with `-Xmx` and `-Xms`.

As noted in [JDK-8349771](https://bugs.openjdk.org/browse/JDK-8349771), these outdated options will soon be deprecated for removal from the java launcher code.

Change has been verified locally, test-fix only, no risk.

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Commit messages:
 - 8349771: Replace usages of -mx and -ms in some monitor tests

Changes: https://git.openjdk.org/jdk/pull/23553/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23553&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349771
  Stats: 38 lines in 2 files changed: 0 ins; 0 del; 38 mod
  Patch: https://git.openjdk.org/jdk/pull/23553.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23553/head:pull/23553

PR: https://git.openjdk.org/jdk/pull/23553