tests/README: warn about zsh incompatibilities.

tests/README: warn about zsh incompatibilities.

* tests/README (User interface, Getting details from failures):
Explicitly state that every test is a "shell script", not just
a "script", and that it's run by `/bin/sh' by default.
* tests/README (User interface, Supported shells): New subsection,
telling about expected portability of the automake test scripts,
describing a zsh incompatibility (w.r.t. $0), and a workaround to
it (with the `--no-function-argzero' option).

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