FIX: Re-apply SignalExtraction test fix

Closes #2380

Original commit: 653f690
Original commit message: update nilearn interface to use runtime.cwd

Reverted in: f4c3db7

Author: oesteban

nipype/interfaces/nilearn.py

@@ -106,7 +106,7 @@ def _run_interface(self, runtime):
                             region_signals.astype(str)))
 
         # save output
-        self._results['out_file'] = os.path.abspath(self.inputs.out_file)
+        self._results['out_file'] = os.path.join(runtime.cwd, self.inputs.out_file)
         np.savetxt(
             self._results['out_file'], output, fmt=b'%s', delimiter='\t')
         return runtime