without trailing slash response will be empty

Clean up all test code (including examples in the documentation)
to use `Client` instead of `supertest.SuperTest<supertest.Test>`.

Remove `createClientForRestServer` because it was not used anywhere
and had the issue of not stopping the server after the test is done.

Add a new helper `createRestAppClient` instead, rework acceptance tests
(including the templates) to use this new helper.

Remove the following dependencie:
  - `mocha`
  - `@types/mocha`
  - `source-map-support`

These dependencies are not needed because `mocha` is invoked via
`lb-mocha` from `@loopback/build`.

Modify the test helper `givenHttpServerConfig` to explicitly set
the port to zero. With this change in place, applications can use
this helper in acceptation tests without having to explicitly
specify `{port: 0}` to override RestServer's default port 3000.

 - @loopback/benchmark@0.2.13
 - @loopback/docs@0.20.0
 - @loopback/example-hello-world@0.10.0
 - @loopback/example-log-extension@0.11.14
 - @loopback/example-rpc-server@0.11.8
 - @loopback/example-soap-calculator@0.4.0
 - @loopback/example-todo-list@0.5.0
 - @loopback/example-todo@0.19.0
 - @loopback/authentication@0.11.14
 - @loopback/boot@0.14.0
 - @loopback/cli@0.24.0
 - @loopback/context@0.12.10
 - @loopback/core@0.11.11
 - @loopback/http-caching-proxy@0.3.9
 - @loopback/http-server@0.4.0
 - @loopback/metadata@0.9.9
 - @loopback/openapi-spec-builder@0.9.1
 - @loopback/openapi-v3-types@0.9.3
 - @loopback/openapi-v3@0.14.4
 - @loopback/repository-json-schema@0.10.14
 - @loopback/repository@0.17.1
 - @loopback/rest@0.24.0
 - @loopback/service-proxy@0.8.5
 - @loopback/testlab@0.13.0