testupdatedata: fix cwd (#15383)

This should allow the inner pytest to pass even when the outer pytest is
invoked outside of the mypy root, e.g.
```shell
cd ..
pytest mypy/mypy/test/testupdatedata.py
```

Addresses
#15283 (comment).

mypy/test/testupdatedata.py

@@ -14,7 +14,9 @@ def _run_pytest_update_data(self, data_suite: str, *, max_attempts: int) -> str:
         Runs a suite of data test cases through 'pytest --update-data' until either tests pass
         or until a maximum number of attempts (needed for incremental tests).
         """
-        p = Path(test_data_prefix) / "check-update-data.test"
+        p_test_data = Path(test_data_prefix)
+        p_root = p_test_data.parent.parent
+        p = p_test_data / "check-update-data.test"
         assert not p.exists()
         try:
             p.write_text(textwrap.dedent(data_suite).lstrip())
@@ -26,7 +28,7 @@ def _run_pytest_update_data(self, data_suite: str, *, max_attempts: int) -> str:
             else:
                 cmd = " ".join(args)
             for i in range(max_attempts - 1, -1, -1):
-                res = subprocess.run(args)
+                res = subprocess.run(args, cwd=p_root)
                 if res.returncode == 0:
                     break
                 print(f"`{cmd}` returned {res.returncode}: {i} attempts remaining")