Fix and enable some tests on Linux (MI)

Summary:
Fix and enable some lldb-mi tests on Linux:

Fixed:
# MiExecInterpreterTestCase.test_lldbmi_thread_step_in
# MiExecTestCase.test_lldbmi_exec_step
# MiStackTestCase.test_lldbmi_stack_info_frame tests.

Also I enabled the following tests on Linux:
# MiExecTestCase.test_lldbmi_exec_step_instruction
# MiSignalTestCase.test_lldbmi_stopped_when_interrupted

All test pass on OS X and Linux.

Reviewers: abidh, vharron, clayborg

Reviewed By: clayborg

Subscribers: vharron, lldb-commits, clayborg, abidh

Differential Revision: http://reviews.llvm.org/D7987

llvm-svn: 231072