Use real test data for fit_a_line model to get valid result

[kuke]

Resolve #11856

('infer shape: ', (10, 1))
('infer results: ', array([[13.366845],
       [13.256419],
       [13.525405],
       [14.875826],
       [13.950964],
       [14.409012],
       [14.067776],
       [13.956516],
       [12.273897],
       [13.826031]], dtype=float32))
('ground truth: ', array([[ 8.5],
       [ 5. ],
       [11.9],
       [27.9],
       [17.2],
       [27.5],
       [15. ],
       [17.2],
       [17.9],
       [16.3]], dtype=float32))