Going to run Karma+Jasmine to test angularjs client with real backend. Since Karma is using its own express but I need to access real nodejs backend with DB and other stuff, I'm thinking on adding interceptor into $httpProvider.interceptors that will just replace my calls to /api and redirec them into real backend location. Is there a better way?
You don't want to do that on unit tests (and personally I wouldn't even do that with E2E tests).
When doing unit tests, the $httpBackend get swapped with a dummy version that is not capable of doing real requests. That is intentional. You shouldn't do any test with the real backend.
On the other hand, there are E2E tests (where you test all your system together) where you could use the real backend in your tests (there are people who likes that, people who doesn't).
Keep in mind that unit test are all about units in isolation, that means that you don't care about dependencies and much much less about a backend.