tests: drop support for older Zsh shells

tests: drop support for older Zsh shells

* defs-static.in: Here.  The fact that such support has been broken for
almost a year (only fixed by today's commit 'v1.12.1-57-gf1e0300'),
causing no bug reports from anyone, shows that such support is not truly
warranted.  And it will get in the way of future improvements in the
handling of the exit trap (because bugs in older Zsh versions will
prevent some of our planned improvements).  So just drop it.
* t/README: Remove obsolete advice for working around bugs in older
versions of Zsh; instead, indicate version 4.3 is the oldest Zsh now
supported.

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