[Dist CI][BE] Run each test in its own process for test_distributed_spawn (#67901)
Summary:
Context: https://github.com/pytorch/pytorch/issues/67061
Use `run_test.py`'s provided flag `"--subprocess"`, passed in like `extra_unittest_args=["--subprocess"]` when running test_distributed_spawn. This will ensure that each test is run separately in its own process. The goal is to more closely simulate how a developer would run a single test when reproducing a CI failure and make reproducibility easier in general.
Also, when a test fails, print out the exact command that was issued so developer knows how to reproduce it.
For example test fails, it will print out something like the following to logs -
```
Test exited with non-zero exitcode 1. Command to reproduce: BACKEND=gloo WORLD_SIZE=3 /fsx/users/rvarm1/conda/envs/pytorch/bin/python distributed/test_distributed_spawn.py -v TestDistBackendWithSpawn.test_Backend_enum_class
```
running test_distributed_spawn is still the same cmd as before:
`
python test/run_test.py --verbose -i distributed/test_distributed_spawn
`
as seen in [distributed contributing](https://github.com/pytorch/pytorch/blob/master/torch/distributed/CONTRIBUTING.md) guide.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67901
Reviewed By: cbalioglu, mruberry
Differential Revision: D32225172
Pulled By: rohan-varma
fbshipit-source-id: 7e8d4c7a41858044bd2a4e0d1f0bf8f1ac671d67
