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

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