Fix parallel testsuite run with Zsh.

Fix parallel testsuite run with Zsh.

This change deals with a Zsh incompatibility in the handling
of the special shell variable `$0' in sourced files; this
incompatibility used to cause utter breakage when the
Automake testsuite was run in parallel mode with Zsh as
the $(TEST_LOG_COMPILER).

For more information, please refer to the thread "Fix parallel
testsuite run with zsh" on automake-patches, dated 2010-12-22:
 <http://lists.gnu.org/archive/html/automake-patches/2010-12/msg00135.html>

This change works around the problems described above for Zsh 4.3
or later, and offers better error messages (instead of random
failures) for earlier Zsh version.

* tests/README (Supported shells): When describing the manual
workaround about the Zsh incompatibility in the handling of `$0',
tell that it is now needed only with Zsh versions preceding 4.3.
Done also some minor rewordings.
* tests/defs-static.in ($argv0): New variable, offers a workaround
for the Zsh incompatibility in the handling of `$0'.
Abort if that variable cannot be correctly set (can happen only
in older Zsh version).
* tests/defs ($me): Define using `$argv0', not `$0'.