Fix broken caffe2 test: PlanExecutorTest.BlockingErrorPlan (#64401)

Fix broken caffe2 test: PlanExecutorTest.BlockingErrorPlan (#64401)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64401

PlanExecutorTest.BlockingErrorPlan uses `ASSERT_DEATH` which internally performs a `fork()`. This can cause problems under certain configurations that use threads. This change updates this test to use the "threadsafe" style for GTest death tests in order to improve its quality in multithreaded environments.

Test Plan:
I confirmed that this change fixes the issue on my devvm with the following command:
```
buck test mode/dev //caffe2/caffe2:caffe2_test_cpu -- PlanExecutorTest.BlockingErrorPlan
```

Reviewed By: praihan

Differential Revision: D30709447

fbshipit-source-id: 12ffd9ad0371e2e5b43a9873c80568e5ab02d246