Testing: Provide simple mocking

[alex3683]

For anyone using the hal-http-client there should be a convenient way to mock responses for requests in a widget spec. As a basis an existing fetch-mock library could be used.
An important feature would be the possibility to wait for a certain request to have been made. The fetch-mock mentioned above allows for naming mocked requests to allow for inspection later. This could also be used to allow for waiting for a given request. Currently some tests need "random" setTimeouts instead to wait for completion ....