test infra cleanup: convert test_runner lib into package

Also does the following:
* adopts PEP8 naming convention for OptionalWith class (now
  optional_with).
* moves test_runner/lldb_utils.py to lldbsuite/support/optional_with.py.
* packages tests in a subpackage of test_runner per recommendations in
  http://the-hitchhikers-guide-to-packaging.readthedocs.org/en/latest/creation.html

Tests can be run from within pacakges/Python/lldbsuite/test via this
command:

  python -m unittest discover test_runner

The primary cleanup this allows is avoiding the need to muck with the
PYTHONPATH variable from within the source files.  This also aids some
of the static code checkers as they don't need to run code to determine
the proper python path.

llvm-svn: 266710

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