Copy the "Skipped tests" section from `tests.md` to `TESTS.md`

[glennj]

The file https://github.com/exercism/bash/blob/main/exercises/shared/.docs/tests.md is used to construct the HELP.md file that is part of the exercism download ... content. It contains a section about what to do with the skip directives.

The file https://github.com/exercism/bash/blob/main/docs/TESTS.md is used for the online docs

The Skipped tests section should appear under Testing Locally.