RFR: 8370315: [IR-Framework] Allow scenarios to be run in parallel [v3]

[Damon Fenacci]

> ## Issue
> Today, the only practical ways to run IR Framework scenarios in parallel seems to be:
> * spawning threads manually in a single test, or
> * letting jtreg treat each scenario as a separate test (the only way to potentially distribute across hosts).
> 
> This makes it a bit cumbersome to use host CPU cores efficiently when running multiple scenarios within the same test.
> 
> ## Change
> This change introduces a method `TestFramework::startParallel` to execute multiple scenarios concurrently. The implementation:
> * launches one task per scenario and runs them in parallel (by default, the maximum concurrency should match the host’s available cores)
> * captures each task’s `System.out` into a dedicated buffer and flushes it when the task completes to avoid interleaved output between scenarios (Note: only call paths within the `compile.lib.ir_framework` package are modified to per-task output streams. `ProcessTools` methods still write directly to `stdout`, so their output may interleave).
> * adds an option `-DForceSequentialScenarios=true` to force all scenarios to be run sequentially.
> 
> ## Testing
> * Tier 1-3+
> * explicit `ir_framework.tests` runs
>   * added IR-Framework test `TestDForceSequentialScenarios.java` to test forcing sequential testing (checkin the output order) and added a parallel run to `TestScenatios.java` (as well as adding `ForceSequentialScenarios` flag to `TestDFlags.java`)
> 
> As reference: a comparison of the execution time between sequential and parallel of all IR-Framework tests using scenarios on our machines (linux x64/aarch64, macosx x64/aarch64, windows x64 with different number of cores, so the results for a single test might not be relevant) gave me an average speedup of 1.9.

Damon Fenacci has updated the pull request incrementally with three additional commits since the last revision:

 - JDK-8370315: use check function
 - Apply suggestion from @chhagedorn
   
   Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
 - JDK-8370315: add exceptions as they arise

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28065/files
  - new: https://git.openjdk.org/jdk/pull/28065/files/c9295971..4e9cc0a2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28065&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28065&range=01-02

  Stats: 16 lines in 2 files changed: 1 ins; 8 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/28065.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28065/head:pull/28065

PR: https://git.openjdk.org/jdk/pull/28065