Fix test_assert_is_detection_link to always raise assert error

[yuyu2172]

Undesired test error from #301
https://travis-ci.org/chainer/chainercv/jobs/247044467

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FAIL: test_assert_is_detection_link (test_assert_is_detection_link.transplant_class.<locals>.C)  parameter: {'link': <test_assert_is_detection_link.InvalidScoreSizeLink object at 0x7f353a392908>, 'valid': False}
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Traceback (most recent call last):
  File "/home/travis/build/chainer/chainercv/tests/utils_tests/testing_tests/assertions_tests/test_assert_is_detection_link.py", line 76, in test_assert_is_detection_link
    assert_is_detection_link(self.link, 20)
AssertionError: AssertionError not raised
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Ran 305 tests in 43.523s
FAILED (failures=1)

[Hakuyume]

Thanks, LGTM