docs($httpBackend): add module declaration for best understanding

According with the Issue #9537. This module declaration in the test is very important. When I started to test in angular I copy and paste this code to see how it works, and I get this `module undefined error`, and just after read some blog posts I figure out that this line is essential for testing your module. So, for best understanding of begginers this can be very helpful.

Closes #9563