Fix round robin sharding (#121022)
Fix round robin sharding when there are no test times and sort_by_time=False
Adds more tests to test_test_selections for sort_by_time=False
Adds more checks to test_split_shards_random for serial/parallel ordering + ordering of tests
Refactoring of dup code
Tested locally by running `python test/run_test.py --shard 3 5` with no test times downloaded and checked that it wasn't an empty list.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121022
Approved by: https://github.com/huydhn, https://github.com/osalpekar
