utils/abtest: Refactor and add bisection method

- Refactor/rewrite most of the code. Also make sure it passes
  pycodestyle/pyflakes now
- Add a new mode that performs bisection on the search space. This
  should be faster in the common case where there is only a small number
  of files or functions actually leading to failure.
  The previous sequential behavior can still be accessed via `--seq`.

llvm-svn: 341679