test/fuzz: fix datetime_strptime fuzzing test

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

(cherry picked from commit a1bd6e0b)
(cherry picked from commit 504a0f88)