tests/*.sh : Switch back to /bin/sh.
Jan Stary reported that on OpenBSD bash isn't located in /bin/ which means that the test fail. He also noted that there didn't seem to be anything bash specific in the tests. This patch takes some suggestions from Jan, plus a few fixes tested on Debian using the bin/sh provided by bash and dash. Suggested-by: Jan Stary <firstname.lastname@example.org>