TST: fixturize test_to_html alignment checks
https://github.com/pandas-dev/pandas/pull/24873#issuecomment-456641936

re-use of fixtures through indirect parametrization.

a hierarchy of fixtures to ultimately create a dataframe fixture containing 16 dataframes of combinations of named and unnamed, single level and multi index independently on the row and columns index.

this dataframe fixture is then reused through indirect parametrization of the dataframe values, the index and column index labels and the index and column index names

both the test functions consist of only two lines

```
result = ...
assert result == ...
```

the diff is a bit cluttered since a function used by the fixtures, `expected_html` has been moved into conftest and converted to a fixture.
