[CI] Run tests in parallel (#4533)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.github/workflows/test.yml

@@ -85,10 +85,7 @@ jobs:
           $CHISEL_FIRTOOL_PATH/firtool -version >> $GITHUB_STEP_SUMMARY
           echo \`\`\` >> $GITHUB_STEP_SUMMARY
       - name: Test
-        run: |
-          ./mill firrtl[_].test
-          ./mill svsim[_].test
-          ./mill chisel[_].test
+        run: ./mill -j0 firrtl[_].test + svsim[_].test + chisel[_].test
       - name: Binary compatibility
         # TODO either make this also check the plugin or decide that we don't
         #      support binary compatibility for the plugin
@@ -181,7 +178,7 @@ jobs:
           dir=$(dirname $(which firtool))
           echo "CHISEL_FIRTOOL_PATH=$dir" >> "$GITHUB_ENV"
       - name: Integration Tests
-        run: ./mill integrationTests[_].test
+        run: ./mill -j0 integrationTests[_].test
 
   # Currently just a sanity check that the benchmarking flow works
   benchmark: