Skipped some duplicated tests.

[gabrieldemarmiesse]

Summary

@taehoonlee
Follow-up on #11037
Still trying to make those tests look younger and prettier.

Related Issues

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)
• This PR is backwards compatible [y/n]
• This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

[gabrieldemarmiesse]

I got a weird error unrelated to my PR:

=================================== FAILURES ===================================
______________________ test_generator_enqueuer_threadsafe ______________________
[gw1] linux2 -- Python 2.7.15 /home/travis/miniconda/envs/test-environment/bin/python
    def test_generator_enqueuer_threadsafe():
        enqueuer = GeneratorEnqueuer(create_generator_from_sequence_pcs(
            DummySequence([3, 200, 200, 3])), use_multiprocessing=False)
        enqueuer.start(3, 10)
        gen_output = enqueuer.get()
        with pytest.raises(RuntimeError) as e:
>           [next(gen_output) for _ in range(10)]
E           Failed: DID NOT RAISE <type 'exceptions.RuntimeError'>
tests/keras/utils/data_utils_test.py:207: Failed

Maybe it's related to #11034 ?

In any case, re-trigerring the tests.

[taehoonlee]

LGTM. And @gabrieldemarmiesse, I would like you to remind that the motivation of these PRs is not only simple style improvements but also CI build time reductions.