Make the debug tests aware of multicore.

Targets now contain an array of harts. When running a regular test, one
hart is selected to run the test on while the remaining harts are parked
in a safe infinite loop.

There's currently only one test that tests multicore behavior, but there
could be more.

The infrastructure should be able to support heterogeneous multicore,
but I don't have a target like that to test with.