Fix ImportError in lldb-mi tests on Linux

Summary:
This patch fixes the following error:
```
 File "/home/llvmbb/llvm-build-dir/lldb-x86_64-clang-ubuntu-14.04/llvm/tools/lldb/test/tools/lldb-mi/signal/TestMiSignal.py", line 5, in <module>
    import lldbmi_testcase
ImportError: No module named lldbmi_testcase
```

Reviewers: vharron, abidh, clayborg

Reviewed By: abidh, clayborg

Subscribers: lldb-commits, vharron, clayborg, abidh

Differential Revision: http://reviews.llvm.org/D7857

llvm-svn: 230466