Introduce a thorough test of the lc_time_tz_convert api checking:
- dates in the distant past - dates in the distant future - daylight saving for different times of the year across the two timezones - historical circumstances (e.g. Pre-timezones US) - Invalid input
The test shows that the current api does not behave consistently in certain circumstances:
- the same time_value is sometimes returned with invalid input - the empty string is sometimes returned with valid input - for dates where the ref-timezones package does not provide information, no result is returned