Enable parallel regression test execution for Windows VS 2022

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.

Co-authored-by: Kiro <kiro-agent@users.noreply.github.com>

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -921,7 +921,7 @@ jobs:
           make CXX=clcache BUILD_ENV=MSVC -C unit test TAGS="[z3]"
           make CXX=clcache BUILD_ENV=MSVC -C jbmc/unit test
       - name: Run CBMC regression tests
-        run: make CXX=clcache BUILD_ENV=MSVC -C regression test
+        run: make CXX=clcache BUILD_ENV=MSVC -j${{env.windows-vcpus}} -C regression test-parallel-jobs
 
   # This job takes approximately 7 to 32 minutes
   windows-msi-package:

jbmc/regression/Makefile

@@ -42,6 +42,19 @@ test-parallel:
 		$(MAKE) "{}" \
 		::: $(DIRS)
 
+# Run all test directories in parallel when invoked with make -j<N>
+# (no GNU Parallel needed).
+# Example: make -j8 test-parallel-jobs
+# Without -j, the directories will run sequentially.
+PARALLEL_DIRS = $(addprefix parallel__,$(DIRS))
+.PHONY: mvn-package test-parallel-jobs $(PARALLEL_DIRS)
+mvn-package:
+	mvn --quiet clean package -T1C
+test-parallel-jobs: $(PARALLEL_DIRS)
+$(PARALLEL_DIRS): parallel__%: mvn-package
+	@echo "Running $*..."
+	$(MAKE) -C "$*" test
+
 
 .PHONY: clean
 clean:

regression/Makefile

@@ -112,6 +112,17 @@ test-parallel:
 		$(MAKE) "{}" \
 		::: $(DIRS)
 
+# Run all test directories in parallel when invoked with make -j<N>
+# (no GNU Parallel needed).
+# Example: make -j8 test-parallel-jobs
+# Without -j, the directories will run sequentially.
+PARALLEL_DIRS = $(addprefix parallel__,$(DIRS))
+.PHONY: test-parallel-jobs $(PARALLEL_DIRS)
+test-parallel-jobs: $(PARALLEL_DIRS)
+$(PARALLEL_DIRS): parallel__%:
+	@echo "Running $*..."
+	$(MAKE) -C "$*" test
+
 
 .PHONY: clean
 clean: