Destroy the attached process in tearDown() to fix ProcessAttachTestCase tests on OS X

This patch fixes the following:
```
1: test_attach_to_process_by_id_with_dsym (TestProcessAttach.ProcessAttachTestCase)
   Test attach by process id ... ok
2: test_attach_to_process_by_id_with_dwarf (TestProcessAttach.ProcessAttachTestCase)
   Test attach by process id ... ok
3: test_attach_to_process_by_name_with_dsym (TestProcessAttach.ProcessAttachTestCase)
   Test attach by process name ... FAILURE
4: test_attach_to_process_by_name_with_dwarf (TestProcessAttach.ProcessAttachTestCase)
   Test attach by process name ... FAILURE

======================================================================
FAIL: test_attach_to_process_by_name_with_dsym (TestProcessAttach.ProcessAttachTestCase)
   Test attach by process name
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/IliaK/p/llvm/tools/lldb/test/lldbtest.py", line 462, in wrapper
    return func(self, *args, **kwargs)
  File "/Users/IliaK/p/llvm/tools/lldb/test/functionalities/process_attach/TestProcessAttach.py", line 35, in test_attach_to_process_by_name_with_dsym
    self.process_attach_by_name()
  File "/Users/IliaK/p/llvm/tools/lldb/test/functionalities/process_attach/TestProcessAttach.py", line 79, in process_attach_by_name
    self.runCmd("process attach -n s" + exe_name)
  File "/Users/IliaK/p/llvm/tools/lldb/test/lldbtest.py", line 2008, in runCmd
    msg if msg else CMD_MSG(cmd))
AssertionError: False is not True : Command 'process attach -n sProcessAttach' returns successfully
Config=x86_64-clang
======================================================================
FAIL: test_attach_to_process_by_name_with_dwarf (TestProcessAttach.ProcessAttachTestCase)
   Test attach by process name
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/IliaK/p/llvm/tools/lldb/test/lldbtest.py", line 479, in wrapper
    return func(self, *args, **kwargs)
  File "/Users/IliaK/p/llvm/tools/lldb/test/functionalities/process_attach/TestProcessAttach.py", line 41, in test_attach_to_process_by_name_with_dwarf
    self.process_attach_by_name()
  File "/Users/IliaK/p/llvm/tools/lldb/test/functionalities/process_attach/TestProcessAttach.py", line 79, in process_attach_by_name
    self.runCmd("process attach -n s" + exe_name)
  File "/Users/IliaK/p/llvm/tools/lldb/test/lldbtest.py", line 2008, in runCmd
    msg if msg else CMD_MSG(cmd))
AssertionError: False is not True : Command 'process attach -n sProcessAttach' returns successfully
Config=x86_64-clang
----------------------------------------------------------------------
```

Failure-x86_64-clang-TestProcessAttach.ProcessAttachTestCase.test_attach_to_process_by_name_with_dsym.log:
```
[...]
runCmd: process attach -n ProcessAttach
runCmd failed!
error: attach failed: more than one process named ProcessAttach:
PID    PARENT USER       TRIPLE                   ARGUMENTS
====== ====== ========== ======================== ============================
43752  43680  IliaK      x86_64-apple-macosx      /Users/IliaK/p/llvm/tools/lldb/test/functionalities/process_attach/ProcessAttach
43663  1      IliaK      x86_64-apple-macosx      /Users/IliaK/p/llvm/tools/lldb/test/functionalities/process_attach/ProcessAttach
[...]
```

llvm-svn: 233272