Skip check_trailing_whitespace test if pep8/pycodestyle is not available

[geimer]

It failed for me with

ERROR: test_check_trailing_whitespace (test.framework.style.StyleTest)
Test for trailing whitespace check.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test/framework/style.py", line 91, in test_check_trailing_whitespace
    result = _eb_check_trailing_whitespace(line, lines, line_number, state)
  File "easybuild/framework/easyconfig/style.py", line 86, in _eb_check_trailing_whitespace
    result = trailing_whitespace(line)
NameError: global name 'trailing_whitespace' is not defined

[boegel]

Missed that indeed, thanks @geimer!