Some excel tests aren't being picked up
Discovered by @abarber4gh 


```
pytest pandas/tests/io/test_excel.py -v
=================================================================== test session starts ====================================================================
platform darwin -- Python 3.6.1, pytest-3.1.0, py-1.4.33, pluggy-0.4.0 -- /Users/taugspurger/Envs/pandas-dev/bin/python3.6
cachedir: .cache
rootdir: /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas, inifile: setup.cfg
plugins: xdist-1.15.0, cov-2.4.0
collected 5 items

pandas/tests/io/test_excel.py::test_styler_to_excel[xlwt] xfail
pandas/tests/io/test_excel.py::test_styler_to_excel[xlsxwriter] PASSED
pandas/tests/io/test_excel.py::test_styler_to_excel[openpyxl] PASSED
pandas/tests/io/test_excel.py::TestFSPath::test_excelfile_fspath PASSED
pandas/tests/io/test_excel.py::TestFSPath::test_excelwriter_fspath PASSED

=========================================================== 4 passed, 1 xfailed in 0.97 seconds ============================================================
```

probably related to the pytest move. PR coming shortly