REGTESTS: converter: add a test for the date formatting converters

REGTESTS: converter: add a test for the date formatting converters

utime, ltime, ms_utime, ms_ltime, us_utime, us_ltime and http_date were
not covered at all. They are checked here against fixed input dates, so
that the expected output is constant, including the offset argument, the
%N specifier with its various widths and the unit argument of http_date.

The local time variants depend on the machine's timezone, so their
output is only matched loosely, which is still enough to verify that the
date and the sub-second part are correct.

The test also verifies that accept_date() and date() remain consistent
with each other when expressed in seconds, milliseconds and
microseconds. A test also checks that the ms_*/us_* converters apply
their offset in the right unit.