Fixup TestStepNoDebug to cleanup properly on shutdown.

This was causing core dumps on MacOSX and was not properly
cleaning up the state of the inferior before exiting.

The test was overriding def tearDown(), but failed to
call the base class after doing its own cleanup.  This
essentially eliminated normal clean-up activity.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@214138 91177308-0d34-0410-b5e6-96231b3b80d8