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

[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.

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Commit messages:
 - JDK-8370315: move declaration
 - JDK-8370315: remove garbage
 - JDK-8370315: add flag and tests
 - JDK-8370315:  revert a few line changes
 - JDK-8370315: move getLastTestVMOutput
 - JDK-8370315: revert removal of lastTestVMOutput
 - JDK-8370315: fix output processing
 - JDK-8370315: [IR-Framework] Allow scenarios to be run in parallel

Changes: https://git.openjdk.org/jdk/pull/28065/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28065&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370315
  Stats: 191 lines in 8 files changed: 139 ins; 2 del; 50 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