tests: drop requirement 'texi2dvi-o'; 'texi2dvi' is enough

tests: drop requirement 'texi2dvi-o'; 'texi2dvi' is enough

* defs: Since we now require Texinfo 4.9 or later in the Automake
generated Makefiles, and since the '-o' option of texi2dvi has been
supported since Texinfo 4.1, the requirement 'texi2dvi-o' is
obsolete -- the simpler requirement 'texi2dvi' is enough.
* t/silent8.sh ($required): Adjust by using simply 'texi2dvi'
instead of 'texi2dvi-o'
* t/txinfo-no-clutter.sh: Likewise.
* t/txinfo13.sh: Likewise.
* t/txinfo23.sh: Likewise.
* t/txinfo24.sh: Likewise.
* t/txinfo25.sh: Likewise.
* t/txinfo28.sh: Likewise.
* t/txinfo33.sh: Likewise.
* t/vtexi4.sh: Likewise.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>