tests: Update Testlib CI tests to use multiheading

These were previously only running on single-threaded machines. Now
they'll be running on 4-core VMs so may as well run tests in parallel.

Change-Id: I7ee86512dc72851cea307dfd800dcf9a02f2f738

.github/workflows/ci-tests.yaml

@@ -152,7 +152,7 @@ jobs:
       - name: Run "tests/${{ matrix.test-dir }}" TestLib quick tests
         id: run-tests
         working-directory: ${{ github.workspace }}/tests
-        run: ./main.py run --skip-build -vv ${{ matrix.test-dir }}
+        run: ./main.py run --skip-build -t $(nproc) -vv ${{ matrix.test-dir }}
 
         # Get the basename of the matrix.test-dir path (to name the artifact).
       - name: Sanatize test-dir for artifact name