regress: Move Texit into hnonfuzz

We are going to want to do this separately in the fuzz playback.  This
avoids difficulties with adnstest (which calls Texit rather than
Hexit) and programs which return from main(), both of which we want to
capture the exit status of in the fuzz playbacks.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>