Use GNU parallel in Windows CI job

This should reduce our test execution time.

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

@@ -767,6 +767,9 @@ jobs:
           New-Item -ItemType directory "C:\tools\cvc5"
           wget.exe -O c:\tools\cvc5\cvc5.exe https://github.com/cvc5/cvc5/releases/download/cvc5-${{env.cvc5-version}}/cvc5-Win64.exe
           echo "c:\tools\cvc5;" >> $env:GITHUB_PATH
+          New-Item -ItemType directory "C:\tools\parallel"
+          wget.exe -O c:\tools\parallel\parallel https://git.savannah.gnu.org/cgit/parallel.git/plain/src/parallel
+          echo "c:\tools\parallel" >> $env:GITHUB_PATH
       - name: Confirm z3 solver is available and log the version installed
         run: z3 --version
       - name: Confirm cvc5 solver is available and log the version installed
@@ -806,7 +809,7 @@ jobs:
           make CXX=clcache BUILD_ENV=MSVC -C unit test TAGS="[z3]"
           make CXX=clcache BUILD_ENV=MSVC -C jbmc/unit test
       - name: Run CBMC regression tests
-        run: make CXX=clcache BUILD_ENV=MSVC -C regression test
+        run: make CXX=clcache BUILD_ENV=MSVC -C regression test-parallel JOBS=${{env.windows-vcpus}}
 
   # This job takes approximately 7 to 32 minutes
   windows-msi-package: