Refactor lldb-mi tests

Summary:
This patch includes following changes:
* split lldb-mi tests into separate folders. It will make our life simpler because we can modify a test program of certain test and don't worry about other tests
* a bit refactoring
* fix comments
* improve some tests

Reviewers: emaste, clayborg, abidh

Reviewed By: clayborg, abidh

Subscribers: clayborg, lldb-commits, emaste, abidh

Differential Revision: http://reviews.llvm.org/D7762

llvm-svn: 230022