Add Mission load-balance option

Without this we need to define how many tests to split apart, adding
this is needed to evenly split tests.

.github/workflows/ci.yml

@@ -190,7 +190,7 @@ jobs:
       - name: Install Dependencies
         run: pnpm install
       - name: Run Tests
-        run: pnpm --filter ${{matrix.workspace}} exec ember exam --parallel=3
+        run: pnpm --filter ${{matrix.workspace}} exec ember exam --parallel=3 --load-balance
 
   build-with-embroider:
     name: ${{matrix.workspace}} Build With Embroider