Added basic support of itegration tests via llvm-lit.

This is just kind of draft.
Probably, it's not a best and/or correct way, but it works, almost :smile: 
To make it working you should make some efforts:
1. Specify path to your built clang at `lit.cfg:7`
2. Specify path to your built plugin (dylib) at `lit.cfg:8`

Now you can run tests with a simple command
```bash
path_to_bin/llvm-lit test/basic.m
```
Expected output:
```
-- Testing: 1 tests, 1 threads --
PASS: KeyPathValidator :: basic.m (1 of 1)
Testing Time: 3.52s
  Expected Passes    : 1
```