ENH: add interval kwarg to get_data_yahoo issue #9071

PR for the issue #9071
The Yahoo! CSV API is the same for 'd', 'w' and 'm' interval values, so testing one is enough to exercise all code in the function. The reason I added a second call with 'v' is that Yahoo! replies with a different CSV format in this case and that assures that it is handled by the same code without errors.

In my opinion it is not particularly useful to add more tests, but if you think it is necessary please tell me and I'll do it.