TST/CLN: deduplicate fixture from test_to_latex.py
this PR is basically just to promote the float_frame fixture to top-level conftest.

once there, there is opportunities to re-use by fixturising current instances of 

```
df = DataFrame(tm.getSeriesData())
```

also allows removing duplicate fixture from pandas/tests/io/formats/test_to_latex.py

so this is just a move for now, fixturization in follow-on PRs