Update test_multi_hot_sparse_categorical_crossentropy to use only MXN…

[kalyc]

Summary

Enable multi_hot_sparse_categorical_crossentropy unit test only for MXNet backend

Related Issues

Fixes https://github.com/awslabs/keras-apache-mxnet/issues/180

PR Overview

• [ ] This PR requires new unit tests [y/n] (make sure tests are included)
• [ ] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [X] This PR is backwards compatible [y/n]
• [ ] This PR changes the current API [y/n]

[kalyc]

CI error: refs/pull/160/head does not point to a valid object!

[kalyc]

close https://github.com/awslabs/keras-apache-mxnet/issues/180