[lit] Send back whole lit.Test object from worker process

In previous commits [1,2] I changed worker.py to only send back the test
result from the worker process instead of the whole test object.  This
was a mistake.  lit.Test contains fields (e.g., xfials, requires,
unsupported) that are only populated when we actually execute the test,
but are queried when we report the results in the parent process.  This
commit essentially reverts the following changes:

[1] a3d2f9b5
[2] 17bb660f