TST/REF: Fixturize constant functions in ConsistencyBase
This PR fixturizes the last constants (`base_functions` and `no_nan_functions`) in ConsistencyBase, and take corresponding tests out of class. 

After this one, only constants in `Base` class needs to be fixed (if `Base` is also wanted to be cleaned)

cc @jreback 
are tests here there any that need marking as slow? if 5s per test function is acceptable, then we are fine for tests in `expanding` i think

i marked two functions as slow in `rolling` one, one is above which replied to @jbrockmendel comment, that test takes 40s, and the other is `test_rolling_apply_consistency` which is involved in the change in this pr, and this takes ~105s. 

generally tests in rolling takes longer and for the rest of tests in rolling, basically all <15s, and in total ~250s, so maybe it's okay to leave them not as slow? @jreback 
 I will double check a bit, but would be nice to get the ballpark for the threshold for slow? 

Update:

other relatively big ones:
`test_rolling_consistency_std`: ~20s
`test_rolling_consistency_var`: ~25s

i also mark them as slow
 emm, the error does not seem related. gentle ping @jreback  thanks @charlesdong1991 very nice!