code: def test_add_trend_t():
      n = 20
      x = np.zeros((20, 1))
      y = add_trend(x, trend="t")
      assert y[0, 1] == 1.0
      assert_array_almost_equal(np.diff(y[:, 1]), np.ones(n - 1))
  
imports: import numpy
  from numpy.random import RandomState
  from numpy.testing import assert_array_almost_equal, assert_array_equal, assert_equal
  import pandas
  from pandas.testing import assert_frame_equal
  import pytest
  from arch.utility.timeseries import ColumnNameConflict, add_trend
label_desc: Use image node_id 10 for calling a global function with example usage: test_add_trend_t() without return types