fix freesurfer test

Author: oesteban

nipype/interfaces/freesurfer/tests/test_preprocess.py

@@ -116,5 +116,5 @@ def test_mandatory_outvol(create_files_in_directory):
     mni2 = freesurfer.MNIBiasCorrection(in_file=filelist[0],
                                         out_file='bias_corrected_output',
                                         iterations=4)
-    assert mni2.cmdline == ('mri_nu_correct.mni --i %s --n 4 --o bias_corrected_output%s'
-                             % (filelist[0], ext))
+    assert mni2.cmdline == ('mri_nu_correct.mni --i %s --n 4 --o bias_corrected_output'
+                             % filelist[0])