BUG: fix infer frequency for business daily
Closes #16624 

Had to update a couple of tests, test_timedelta and test_business_daily, which appear to rely on the old incorrect behaviour. Added a new regression test, test_business_daily_look_alike, for the bug.

Modest performance impact: for 50,000-long business daily index, infer_freq goes from 2.54 ms to 3.91 ms. There should be no performance impact in most cases for non-business daily index, as the initial day_deltas check will throw out most of these cases.