Clean lldb-mi test cases.

This patch adds lldbmi_testcase.MiTestCaseBase class and removes
a lot of repitition in the lldb-mi test cases. Also cleans import
list and code-style.

Presented for review in http://reviews.llvm.org/D7162.
Patch from Ilia K <ki.stfu@gmail.com>.

llvm-svn: 227218