TST: assert that IncompatibleFrequency is raise (rather than ValueError)

[jreback]

#13079 (comment)

just need to make sure that we are raising IncompatibleFrequency as appropriate (and testing explicity for this rather than ValueError) when constructing Periods.

[jreback]

#13250 prob convers most/all of this.

[sinhrks]

Yes, covered in #13250.

[jreback]

thanks