Require texi2dvi in tests, makeinfo may not be enough.

Require texi2dvi in tests, makeinfo may not be enough.

* tests/txinfo16.test: RHEL 5.2 has makeinfo and texi2dvi in
separate packages, so also list the latter as required, as the
test generates DVI and/or PDF output (through distcheck).
* tests/txinfo18.test: Likewise.
* tests/txinfo21.test: Likewise.
* tests/txinfo22.test: Likewise.
* tests/txinfo3.test: Likewise.
* tests/version7.test: Likewise.
* THANKS: Update.
Report by Alexander Martens.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>