code: def test_use_backend_str(self):
      with fastmath.use_backend("tensorflow-numpy"):
          self.assertEqual(fastmath.backend_name(), "tensorflow-numpy")
  
imports: import collections
  from absl.testing import parameterized
  import gin
  import jax.numpy
  import numpy
  from tensorflow import test
  from trax import fastmath
label_desc: Use image node_id 10 for calling the BackendTest obj's underlying member method code with example usage: obj.test_use_backend_str() without return types