Bug 1229233 - Add a (incomplete) end-to-end build test. r=gps

We have many unit tests in the tree for some small parts of the build system
pipeline, but we don't have anything that resembles an end to end test, and we
kind of rely on the resulting Firefox not being broken by our changes.

With the Faster make backend growing, I want to ensure it produces the same
thing as the recursive make backend, at least for the parts it supports.

This adds some kind of test that allows to check that.