Additional test helpers in Cake.Testing to ease unit-testing of Cake builds

[ap0llo]

I have been writing unit tests for some Cake.Frosting-based builds (specifically tests for tool aliases and Frosting tasks).

For that I've been using the Cake.Testing package that provides some "Fake" implementations of core Cake functionality (like FakeLog).

However, there are some things, for which no default fake is included which I had to add to the test project.
Specifically, I needed

• FakeProcessRunner: A implementation of IProcessRunner that records all started processes and allows customizing the exit codes/outputs of the process (this mostly reuses FakeProcess)
• FakeContext: A fake implementation of ICakeContext which is using fake implementations for most properties (e.g. FakeEnvironment for environment)

I think it would be useful to include this functionality in Cake.Testing.

Would you be open to include that functionality, even if it is not needd for testing Cake itself?
I would be happy to prepare a PR for this.