If not defined it was raising a KeyError. Use the 'get' function to
handle non defined value.

It did not put the value in the `default` case as it would have changed
the behavior when `RIOTBASE` is defined but empty.

As testrunner is moved to dist/pythonlibs which is exported
via PYTHONPATH, testrunner is found by all test scripts.

The wait before board is started is 3 by default but can be overwritten by
setting 'TESTRUNNER_START_WAIT' environment variable.

The object-like access to frame information in the traceback was only
introduced in Python 3.5. Before that version it was a 4-tuple [[1]].

The indexed way to access the frame seems to be upwards-compatible for
newer versions (tested with python 3.5, maybe some of the Arch crew can
test with even newer versions), so I used that one.

[1]: https://docs.python.org/3.4/library/traceback.html#traceback.extract_tb