[lldb-mi] Fix bugs in target-select-so-path.test

Summary:
* This patch fixes hanging of the test in case of using python3, changes callback
  function that will be called if the timer ends, changes python interpreter to
  `%python` that is set up by llvm-lit.
* Also, the test didn't work properly since it didn't contain a call of
  filecheck_proc.communicate(), that means that filecheck didn't run and its
  return code was equal to 0 in all cases.

Reviewers: teemperor, labath, tatyana-krasnukha, aprantl

Reviewed By: teemperor, labath

Subscribers: ki.stfu, lldb-commits

Differential Revision: https://reviews.llvm.org/D52498

llvm-svn: 343033