Increase default test timeout for distributed tests (#80330)
When running on clusters the startup time for the subprocesses might be much higher which leads to spurious failures.
So increase this to 300s similar to torch/testing/_internal/distributed/distributed_test.py
Also introduces `DISTRIBUTED_TESTS_DEFAULT_TIMEOUT` as suggested by @malfet in #55896
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80330
Approved by: https://github.com/malfet
