[Analyzer] Refactor analyzer testing scripts.

 - Exporting needed function for future reuse.
 - Idiomatic python: using with `file as f` instead of `try/finally`.
 - Fixing some indentation issues.
 - No need to reinvent python `multiprocessing.getCPUCount()`
 - Removing a function parameter which is always the same under all invocations.
 - Adding some docstrings.

Differential Revision: https://reviews.llvm.org/D38156

llvm-svn: 313949