CI: Do not run mypy_primer on stubtest/stubgen PRs (python#12295)

PRs that only affect stubtest or stubgen are never going to have any effect on mypy_primer's output, so we can safely skip the CI check for those PRs. The same goes for PRs that only alter files in the tests directory.

.github/workflows/mypy_primer.yml

@@ -8,6 +8,10 @@ on:
     - '**/*.rst'
     - '**/*.md'
     - 'mypyc/**'
+    - 'mypy/stubtest.py'
+    - 'mypy/stubgen.py'
+    - 'mypy/stubgenc.py'
+    - 'mypy/test/**'
 
 jobs:
   mypy_primer: