[lit] Run unit tests as part of lit test suite

Summary:
The Python file `utils/lit/lit/ShUtil.py` contains:

1. Logic used by lit itself
2. A set of unit tests for that logic, which can be run by invoking
  `python utils/lit/lit/ShUtil.py`

Move these unit tests to a `tests/unit` subdirectory of lit, and run
the tests as part of lit's test suite. This ensures that, should the
lit test suite be included in LLVM's own regression test suite, these
unit tests will also be run.

(Instructions on how to run lit's test suite can be found in
`utils/lit/README.txt`.)

Reviewers: ddunbar, echristo, delcypher, beanz

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D25411

llvm-svn: 283968