Enable parallel regression test execution for Windows VS 2022

[tautschnig]

Add a 'test-parallel-jobs' Makefile target to both regression/Makefile and jbmc/regression/Makefile that uses make -j for parallelism instead of GNU Parallel (which is unavailable on Windows runners).

The target creates a phony rule per test directory using the $(addprefix parallel__,$(DIRS)) pattern, allowing make's built-in job server to schedule directories in parallel.

The VS 2022 CI job is updated to use this target with -j$windows-vcpus, replacing the sequential 'make test' invocation. This reduces test wall-clock time from 40 minutes down to 15 minutes.

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• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[Copilot]

Pull request overview

Enable parallel execution of regression tests on Windows VS 2022 runners by adding a Make-only parallel test target (avoiding GNU Parallel) and switching the CI job to use it.

Changes:

• Add test-parallel-jobs targets that schedule test directories via make’s jobserver (make -j).
• Add per-directory phony targets (parallel__<dir>) to support directory-level parallel execution.
• Update the VS 2022 CI workflow to run regression tests with -j${{ env.windows-vcpus }} using the new target.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
regression/Makefile	Adds test-parallel-jobs and parallel__* targets to run test directories in parallel via make jobserver.
jbmc/regression/Makefile	Adds analogous test-parallel-jobs target, with a Maven build step before running per-directory tests.
.github/workflows/pull-request-checks.yaml	Switches Windows regression step from sequential make test to parallel make -j... test-parallel-jobs.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.01%. Comparing base (5719027) to head (d9c842e).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8856      +/-   ##
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- Coverage    80.02%   80.01%   -0.01%     
===========================================
  Files         1700     1700              
  Lines       188345   188345              
  Branches        73       73              
===========================================
- Hits        150716   150703      -13     
- Misses       37629    37642      +13     

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