Initial implementation of parallel type checking

.github/workflows/test.yml

@@ -101,6 +101,18 @@ jobs:
         #   tox_extra_args: "-n 4 mypyc/test/test_run.py mypyc/test/test_external.py"
         #   debug_build: true
 
+        - name: Parallel tests with py314-ubuntu, interpreted
+          python: '3.14'
+          os: ubuntu-24.04-arm
+          toxenv: py
+          tox_extra_args: "-n 4 --mypy-num-workers=4 mypy/test/testcheck.py"
+        - name: Parallel tests with py314-ubuntu, mypyc-compiled
+          python: '3.14'
+          os: ubuntu-24.04-arm
+          toxenv: py
+          tox_extra_args: "-n 4 --mypy-num-workers=4 mypy/test/testcheck.py"
+          test_mypyc: true
+
         - name: Type check our own code (py39-ubuntu)
           python: '3.9'
           os: ubuntu-latest