Allow initializing TestCase() outside of unittest.main() (#39695)
Summary:
When debugging it is sometimes useful to call test code manually. This change makes that easier.
Before this change, one would get the following error:
```
$ python -c "from torch.testing._internal.jit_utils import JitTestCase; JitTestCase()"
Traceback (most recent call last):
File "<string>", line 1, in <module>
File "/home/jansel/pytorch/torch/testing/_internal/common_utils.py", line 740, in __init__
test_method = getattr(self, method_name)
AttributeError: 'JitTestCase' object has no attribute 'runTest'
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39695
Test Plan: `python -c "from torch.testing._internal.jit_utils import JitTestCase; JitTestCase()"`
Differential Revision: D21959249
Pulled By: jansel
fbshipit-source-id: 8435249f102338c957c3a7a7aad48d21d372a8cf
