Add tests for new functions.

[kevindecapite]

The implemented missing functions:

• swe_heliacal_ut
• swe_heliacal_pheno_ut
• swe_vis_limit_mag

...are untested.

A test should compare C output and php output, for the same input parameters.

[aloistr]

I suggest this is postponed, it has low priority because these functions are not used.