tests: fix typo in test_special_fields
tests: fix typo in test_special_fields

timestap -> timestamp.  It was working because a previous timestamp was
present.

bjackman/trappy,sinkap/trappy,joelagnel/trappy,JaviMerino/trappy,bjackman/trappy,joelagnel/trappy,ARM-software/trappy,bjackman/trappy,ARM-software/trappy,sinkap/trappy,ARM-software/trappy,sinkap/trappy,ARM-software/trappy,JaviMerino/trappy,joelagnel/trappy,bjackman/trappy,joelagnel/trappy,JaviMerino/trappy,sinkap/trappy