Fix the endianness issue in `//tensorflow/python/framework:tensor_util_test` on s390x
Hi @cantonios ,

Could you please review this PR when you have some time?

Thank you very much!
Test case `//tensorflow/python/framework:tensor_util_test` failed on s390x (big-endian arch) because in `testHalf()` function the `tensor_content` data which is used for comparison with the tensor proto string is hard-coded in little-endian format.

This PR followed the existing pattern in `testFloatMutateArray()` function to add an endianness check in `testHalf()` function and choose the corresponding `tensor_content` data which is in consistent with the platform endianness.

The above mentioned test case will pass on both little-endian and big-endian systems after applying the code change.