add more unittests

[reedjc]

Add unittests for the following functions:

dnstable_dupsort_func (this needs to be documented too, see ticket #58)
dnstable_entry_format
dnstable_formatter_destroy
dnstable_formatter_init
dnstable_formatter_set_date_format
dnstable_formatter_set_output_format
dnstable_formatter_set_rdata_array
dnstable_query_set_aggregated
dnstable_query_set_offset
dnstable_reader_init_setfile
dnstable_reader_reload_setfile