title: pytest using fixtures as arguments in parametrize
body: <p>I would like to use fixtures as arguments of <code>pytest.mark.parametrize</code> or something that would have the same results.</p>
  
  <p>For example:</p>
  
  <pre><code>import pytest
  import my_package
  
  @pytest.fixture
  def dir1_fixture():
      return '/dir1'
  
  @pytest.fixture
  def dir2_fixture():
      return '/dir2'
  
  @pytest.parametrize('dirname, expected', [(dir1_fixture, 'expected1'), (dir2_fixture, 'expected2')]
  def test_directory_command(dirname, expected):
      result = my_package.directory_command(dirname)
      assert result == expected
  </code></pre>
  
  <p>The problem with fixture params is that every param of the fixture will get run every time it's used, but I don't want that. I want to be able to choose which fixtures will get used depending on the test.</p>