Extract track-specific test instructions from `config/exercise_readme.go.tmpl`

[ErikSchierboom]

Each track needs a file that contains track-specific instructions on how to manually run the tests. The contents of this document are only presented to the student when using the CLI. This file lives at exercises/shared/.docs/tests.md. You almost certainly already have this information, but need to move it to the correct place.

For v2 tracks, this information was (usually) included in the readme template found at config/exercise_readme.go.tmpl. As such, tracks can extract the test instructions from the config/exercise_readme.go.tmpl file to the exercises/shared/.docs/tests.md file.

See https://github.com/exercism/csharp/pull/1557/files for an example PR.

Tracking

https://github.com/exercism/v3-launch/issues/51