docs(hiptensor): Add TESTING.md with testing information (#10434)

## Motivation

Documentation with information on how to run tests and what is tested.
It's a first revision of the documentation based on a general TESTING.md
template for ROCm libraries.

## Submission Checklist

- [X] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

ISSUE ID : AIHIPTENS-554