Rename and/or move test_utils.py

[yaoshiang]

🐛 Bug

This name will generally be understood as testing a utils.py. Unfortunately, this file is actually a holder of utilities for testing.

Under its current naming, test_utils.py will be picked up by testing tools like pytest and unittest.

Expected behavior

test_utils.py can either go into a separate module, or, at the very least, not follow the "test_*.py" naming format, e.g., it could go in a xla/test/utils.py file, or, xla/test_utils/utils.py file.

[ysiraichi]

That makes sense to me. torch_xla/test/utils.py sounds like a good candidate.