This is a follow-up of #34454. 'datetime' extension module attempts to encode input strings into UTF-8 in several places, which requires special care because some valid Python strings can't be represented in UTF-8. It makes sense to add more tests for methods dealing with strings.

Note that my PR doesn't attempt to deal with #34481. In cases where behavior differs between C and Python datetime impls the tests check only for  absence of crashes.