tests of native (C) command if built

Hello,

I have changed the texi2any commands tests such that if the ctexi2any
native command built from C is built, it is tested along with the Perl
texi2any command, which is always tested.

Tell me if you think that it should be conditional, for example based on
--enable-using-c-texi2any and/or --enable-additional-checks.

-- 
Pat