Function `datetime_strptime` decodes string with datetime according to specified format, it accepts a datetime struct, buffer with datetime and string with format in arguments. Fuzzing test used static string "iso8601" as a format and it blocked fuzzing test to cover functions used by datetime_strptime under the hood. Fuzz introspector shows that code coveraged by a test is quite low. Patch updates the test to make it more effective: buffer with datetime and format string are generated using FDP (Fuzzing Data Provider). Test file extension was changed to .cc, because FuzzingDataProvider is used and we need building it by C++ compiler. Function `tnt_strptime` uses assert, that triggered by fuzzing tests. Therefore it was replaced with to if..then. 1. https://storage.googleapis.com/oss-fuzz-introspector/tarantool/ Fixes #8490 NO_CHANGELOG=fuzzing test NO_DOC=fuzzing test NO_TEST=fuzzing test

Fixes #8502 Needed for #8490 NO_DOC=bugfix NO_TEST=covered by fuzzing test

Part of tarantool/tt#286 NO_TEST=third-party dependency NO_DOC=third-party dependency

Follows up #8488 NO_CHANGELOG=testing NO_DOC=testing NO_TEST=testing

Follows up #8488 NO_CHANGELOG=testing NO_DOC=testing NO_TEST=testing