Fix an AttributeError in dotest.py if --executable points to a wrong place

This patch fixes the following case:
```
  $ ./dotest.py --executable=~/p/llvm/build_ninja/bin/lldb tools/lldb-mi/
  '~/p/llvm/build_ninja/bin/lldb' is not a path to a valid executable
  Traceback (most recent call last):
    File "./dotest.py", line 1306, in <module>
      setupSysPath()
    File "./dotest.py", line 1004, in setupSysPath
      if not lldbtest_config.lldbExec:
  AttributeError: 'module' object has no attribute 'lldbExec'
```

And with this fix:
```
  $ ./dotest.py --executable=~/p/llvm/build_ninja/bin/lldb tools/lldb-mi/
  '~/p/llvm/build_ninja/bin/lldb' is not a path to a valid executable
  The 'lldb' executable cannot be located.  Some of the tests may not be run as a result.
```

llvm-svn: 247256