Add README entry about test.File

[FollowTheProcess]

Description

Motivation and Context

Closes #

How Has This Been Tested?

Checklist

• [ ] I have updated the documentation if needed.
• [ ] I have updated the tests if needed.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (13cf44e) 93.25% compared to head (92c09d6) 93.25%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #13 +/- ## ======================================= Coverage 93.25% 93.25% ======================================= Files 1 1 Lines 89 89 ======================================= Hits 83 83 Misses 4 4 Partials 2 2 ```

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