CI: codecov for multiprocessing (#30432)

old-commit-hash: 26294173207cd5e3c586ca71a88fdd3a09fc7345

.github/workflows/selfdrive_tests.yaml

@@ -213,6 +213,7 @@ jobs:
       run: |
         ${{ env.RUN }} "CI=1 coverage run selfdrive/test/process_replay/test_processes.py -j$(nproc) && \
                         chmod -R 777 /tmp/comma_download_cache && \
+                        coverage combine && \
                         coverage xml"
     - name: Print diff
       id: print-diff
@@ -282,6 +283,7 @@ jobs:
       run: |
         ${{ env.RUN_CL }} "unset PYTHONWARNINGS && \
                            ONNXCPU=1 CI=1 NO_NAV=1 coverage run selfdrive/test/process_replay/model_replay.py && \
+                           coverage combine && \
                            coverage xml"
     - name: Run unit tests
       timeout-minutes: 4