Relax text comparison for numpy formatting of arrays

* Previously, strong assumptions were made about how numpy.ndarrays
  are formatted as strings. This led to breakages due to certain
  unclear changes in numpy or its dependencies. This CL relaxes the
  assumption and fix the affected tests for tfdbg and eager.
* The tests in tensor_format_test.py are simplified through helper
  methods.

PiperOrigin-RevId: 181494182