docs(guide/unit-testing): improve unit testing guide

This commit adds to the unit testing guide:

- an explicit section on additional libraries: Karma, Jasmine and
  angular-mocks and link to the docs for those projects too. Explain the
  benefit and use case for each of these libaries
- fully featured test examples and add more documentation
  around them, in particular the controller test
- a clear separation between the section on principles of testing
  and the actual tutorial on writing a test

Closes #8220