[lit] Use better name for "test in parallel" concept

[lit] Use better name for "test in parallel" concept

In the past, lit used threads to run tests in parallel. Today we use
`multiprocessing.Pool`, which uses processes. Let's stay more abstract
and use "worker" everywhere.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D68475

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373794 91177308-0d34-0410-b5e6-96231b3b80d8