Refactor test running to improve parallelism

Before this change, we would ensure that tests for a package were run
before anything used that package, even just to compile. With this
change, we'll let other packages compile, but still run the tests in
dependency order, and before the binaries are finally installed.

This shrinks our initial build time from ~9 seconds to ~5 seconds on my
machine. Most of the improvements come from building the "real" package
and the test package in parallel.

Change-Id: I935670feecadacbf09b250bcaa1659a3599edc58