TST: increase coverage of asfreq
In order to  increase coverage of `asfreq` a test is added, in which we pass `freq = '2M'` to `asfreq`. There's no test which has a number with the frequency yet.
@MarcoGorelli, could you please take a look at this pr? Seems like ci failure isn't related to my changes. just to clarify for anyone passing by - the issue is that existing tests were all like "freq='M'", rather than "freq='2M'". adding such a test increases our confidence in https://github.com/pandas-dev/pandas/pull/52064