Developer's Guide: best practices for shell scripts

Discussion for this started in !1048 (comment 182423). Copying from there:

it's something we should retain in long term. Many of the comments are really recommendations that are valid for future tests. Can you add them to the Developer's guide?

I was thinking either about a section in doc/devel/unit-tests.dox or perhaps a separate file if you feel that shell tests merit their own. But please don't spend too much time on it. Improving developer's guide is a rabbit hole that many fallen into...

Assigning to 1.9.4 because it was meant for 1.9.3 but did not make it in.