Add interface to get run and test deps

[Javier-varez]

Since tests are identified as a single command, dependencies for tests and run are implicitly assumed to be the same as for the Build step, including the target binary. This is, however not correct because it doesn't account for the fact that we may want to use a different test runner or some additional target that must be compiled before testing, but not necessarily in the build step of the target.

With this purpose this change introduces an extendedTestInterface and extendedRunInteface where the user may also specify a TestDeps() or RunDeps() function to get the dependencies of the test/run step.

I have made the decision of creating a new interface rather than extending the exiting one because:
a) Not all targets may have additional dependencies.
b) By creating a breaking change in the interface we could potentially miss some run/test targets that now don't satisfy the new interface and therefore it is silently ignored. I find this is a bit of a design issue in rules (or in golang, actually) that does not allow us to say: I want this target to implement interface X, and rather relies on the methods existing.