[lit] Add back LitTestCase

This essentially reverts a commit [1] that removed the adaptor for
Python unittests.  The code has been slightly refactored to make it more
additive: all code is contained in LitTestCase.py.

Usage sites will require a small adaption:
```
[old]
  import lit.discovery
  ...
  test_suite = lit.discovery.load_test_suite(...)

[new]
  import lit.LitTestCase
  ...
  test_suite = lit.LitTestCase.load_test_suite(...)
```

This was put back on request by Daniel Dunbar, since I wrongly assumed
that the functionality is unused.  At least llbuild still uses this [2].

[1] 70ca752c
[2] https://github.com/apple/swift-llbuild/blob/master/utils/Xcode/LitXCTestAdaptor/LitTests.py#L16

Reviewed By: ddunbar

Differential Revision: https://reviews.llvm.org/D69002

llvm-svn: 374947