Spike: migrate tests to Jest (ts-jest)

[bajtos]

Let's try to speed up our dev process and CI builds by migrating to ts-jest.

1. Jest can run tests in parallel and provides clever watch mode.
2. ts-jest compiles TypeScript files on the fly, removing the need for costly build.
3. Jest provides tools for snapshot-based testing, this would be very useful to test CLI code scaffolding TypeScript files and to test code emitting OpenAPI spec.

Known problems to resolve:

• @loopback/metadata relies on instanceof Function check that does not work under Jest, see fix(metadata): don't rely on instanceof checks #3160
• @loopback/boot is not able to find & load artifacts from original .ts files
• Tests using classes defined outside of the scope of test functions fail with ReferenceError, e.g. ReferenceError: Cannot access 'CustomerController' before initialization.
• Integration with express is broken
• Repository bindings are not picked up (The key 'repositories.NoteRepository' is not bound to any value in context application)

See the list of skipped files in https://github.com/strongloop/loopback-next/blob/test/jest/jest.config.js for the up-to-date information.

[bajtos]

Work started in https://github.com/strongloop/loopback-next/tree/test/jest