class_docstr: Test private conversions between 'z' and 'z**-1' polynomials.
code: def test_full(self, xp, scp):
      # Numerator and denominator same order
      num = [2, 3, 4]
      den = [5, 6, 7]
      num2, den2 = scp.signal.TransferFunction._z_to_zinv(num, den)
      num3, den3 = scp.signal.TransferFunction._zinv_to_z(num, den)
  
      # convert to 1D arrays to help the comparator
      return xp.atleast_1d(num2, den2, num3, den3)
  
imports: from math import sqrt, pi
  from pytest import raises
  import cupy
  from cupy import testing
  import cupyx.scipy.signal
label_desc: Use image node_id 1 for calling the TestTransferFunctionZConversion obj's underlying member method code with example usage: obj.test_full(xp, scp) and returns: xp