add some mock/stub guidelines to testing guideline (#16466)

style_guides/backend.md

@@ -28,3 +28,8 @@ We use [revive](https://github.com/mgechev/revive) as a go linter, and do enforc
 We use GoConvey for BDD/scenario based testing. Which we think is useful for testing certain chain or interactions. Ex https://github.com/grafana/grafana/blob/master/pkg/services/auth/auth_token_test.go
 
 For smaller tests its preferred to use standard library testing.
+
+### Mocks/Stubs
+As a general rule of thumb we try to override/replace functions/methods when mocks/stubs are needed. One common task is the need of overriding time (`time.Now()`). See usage of `getTime` variable in [code](https://github.com/grafana/grafana/blob/52c39904120fb0b98494b961be67bb47574245b1/pkg/services/auth/auth_token.go#L22) and in [test](https://github.com/grafana/grafana/blob/52c39904120fb0b98494b961be67bb47574245b1/pkg/services/auth/auth_token_test.go#L23-L26) as an example.
+
+When you need to stub/mock an interface you can implement a struct that allows you to override methods on a test-by-test basis. See [stub](https://github.com/grafana/grafana/blob/52c39904120fb0b98494b961be67bb47574245b1/pkg/services/auth/testing.go) and [example usage](https://github.com/grafana/grafana/blob/52c39904120fb0b98494b961be67bb47574245b1/pkg/middleware/middleware_test.go#L153-L180).