Move testing infrastructure to llvm-lit and completly remove all DejaGNU support

This work changes our testing infrastructure to llvm-lit. This has many advantages

* Once we re-structure the test suite (need to put each test in its own folder so multiple running copies of KLEE don't clobber their ``klee-last`` folders) we can run the test suite in parallel and be able to run tests super fast :)
* Remove DejaGNU dependency. It now means we depend on python, but we already did previously anyway.
* Vastly superior test output when tests fail (use ``-v`` flag)
* Easily run tests (or directories of tests) individually by doing something like

```
$ cd KLEE_BIN_DIR/test
$ llvm-lit -v -s Feature/DoubleFree.c
```

I'm not sure if this breaks #64 . I've not tried yet.