tests: avoid several spurious failures on Solaris

tests: avoid several spurious failures on Solaris

* t/ax/is_newest: Rewrite to be Bourne-compatible, for /bin/sh shells like
Solaris' that are not POSIX-conforming.  The script is so small that such
a rewrite is easier than going through the hoops that would be required to
ensure this script is always executed with a POSIX shell.
* t/ax/is: Add a comment stating that this script is to be kept Bourne
compatible as well.

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