docs: Testing your extension

[bajtos]

Overview

Thinking in LoopBack explains how to write tests for as part of app development. We should create a similar content explaining extension developers how to write a good test suite.

Acceptance Criteria

• Verify if the Thinking In LoopBack docs provide a solid approach for testing your components/extensions.
• Extend "Writing Extensions" to describe how to test:
  • providers
  • controllers
  • repositories
  • SequenceActions (REST-specific)
  • mixins (adding application methods/configuration)
    • how to test controllers/repositories provided by the mixin
    • testing to ensure your mixin is available to other components (app.component(...) API)
    • introducing new binding sugar functions (app.foo(...)) see feat(repository): Introduce Mixin to support repo binding #541 for an example

See also #525 and #553

[dhmlau]

Marking as p1 because i believe this is a must-have for beta.

[dhmlau]

Discussed with @bajtos, will make this as a stretch goal for Sprint 48 (start the work on Sprint 48 and aim to finish on Sprint 49)