Add and update lldb-mi tests.

This patch includes following changes:

Fix comments and code style
Add new tests for many commands
Improve existing tests
Merge MiProgramArgsTestCase and MiExecTestCase
Improve runCmd of MiTestCaseBase: add exactly option
Improve test example (make it more complicated)

Patch from ki.stfu. I xfailed some tests on Linux and also
added an empty command after -gdb-exit as it blocks without
it.

Patch was reviewed in http://reviews.llvm.org/D7410.

llvm-svn: 228286