The _$httpBackend_
has nothing to flush because you don't make any http request in your test.
You need to invoke some code that make an http request.
Then, once something somewhere made an http request in your test, you can call the flush
method so that a response is provided for the request that has been made.
Something like:
it('should fetch list of users', function(){
// Do something that will make an http request
MyService.getAllUser(function ...) // for example
// Then provide a response for the http request that
// has been made when getAllUser was called
$httpBackend.flush();
// Check the expected result.
expect(something).toBe('Bob');
});