testlab: add helpers for testing HTTP servers

[bajtos]

• Move code from packages/loopback/test/support to packages/testlab

• Drop Client class in favour of supertest

• Add helper APIs for setting up server + supertest:

  • Given a LoopBack app, start it on an ephemeral port (listen(0)), wait until it's listening and then return a supertest instance pointing to this server. (This is useful for testing LoopBack apps.)
  • Given a low-level http.Server instance, start it on an ephemeral port (listen(0)), wait until it's listening and then return a supertest instance pointing to this server. (This is useful e.g. for testing SwaggerRouter in isolation.)

  These two helpers should share most of the implementation code.

Background info from #135:

• Rework tests to use supertest instead of request. Request's interface is already showing limitations, it's time to move to something better while the amount of needed changes is low.
• Consider adding supertest to testlab, this makes sense when we also add helpers for setting up HTTP server for a loopback app/router and returning a supertest instance connected to that server.

cc @ritch @superkhau

[superkhau]

+1 and p1ing this as it will help with so many tests.

[ritch]

+1 for removing request blockers so we can user helpers... but we should have our own wrapper that is the equiv of supertest for lbn apps.

[ritch]

I would change the approach to this:

Add helper APIs for setting up server + supertest:

instead we should do

Add helper APIs for setting up server + superagent:

[bajtos]

I would change the approach to this:

Add helper APIs for setting up server + supertest:

instead we should do

Add helper APIs for setting up server + superagent:

@ritch let's discuss. Why do you perfer superagent over supertest?

What I like about supertest is the addition of .expect() API, which makes it easier to write assertions in the tests.

Compare supertest-based code

return supertest(app).get('/foo')
  .expect(200)
  .expect('Content-Type', /json/);
  .expect({ok: true});

with superagent based code

const result = await supertest(app).get('/foo');
expect(result).to.have.property('statusCode', 200);
expect(result.headers).to.have.property('content-type').match(/json/);
expect(result.body).to.deepEqual({ok: true});

[superkhau]

Do they both take Express apps? That makes it interesting because our app may not work OOB?