TST: add test with freq=2M to increase coverage asfreq
xref #52064

Added a test in which a Series has a PeriodIndex with `freq=2M` to increase coverage `asfreq`. 
The reason: existing tests don't cover a case if the index of a Series is a PeriodIndex with `freq=2M`, then asfreq converts time series to a new one with the frequency `freq=M`.
@MarcoGorelli, could you please take a look at this pr?