fix TestGFM.test_hilite to use has_pygments
fix TestGFM.test_hilite to use has_pygments

I also refer testcase of `python-markdown`.
https://github.com/waylan/Python-Markdown/blob/e594213ab689847ee7d9654817b8fa80fafb0fb7/tests/test_extensions.py#L386-L388

Zopieux/py-gfm,googlearchive/py-gfm