title: pytest test parameterization override
body: <p>I am currently parametrizing all of my testcases using <code>pytest_generate_tests</code> and this works well. </p>
  
  <p>What I'd like to do now is override this behavior for a specific test. 
  If I try and use the <code>pytest.mark.parametrize</code> decorator on the test itself, I get a <code>ValueError: duplicate</code> error which is understandable as I'm now trying to parametrize the test in two places. </p>
  
  <p>Is there a way I can override the "default" parameterization for this one test case? </p>
  
  <p>I can achieve this by doing something like the below but its a very hacky way to do it:</p>
  
  <pre><code>def pytest_generate_tests(metafunc):
      fixture_modes = ['mode1', 'mode2']
      if 'fixture' in metafunc.fixturenames:
          fixture  = metafunc.config.getoption('fixture')
          if fixture:
              fixture_modes = [fixture]
          if metafunc.function.__name__ != 'func_to_skip':
              metafunc.parametrize('fixture_mode', fixture_modes, indirect=True)
  </code></pre>
  
  <p>Is there a better way to do this?</p>