Test authoring tutorial/info page

It'd be great to have some guidelines/information on authoring and modifying tests in Moore. We already have a section on running tests but it's not obvious how to add new ones, if there are conventions, etc.

@rmatev had some ideas on !339 (merged) on how to expect warnings which should be included in the page:

1. never expect/ignore errors or fatals (we don't want "severity inflation" for these)
2. if a warning is supposed to be ignored irrespective of the particular job configuration, then put it in the central list
3. if a warning should be ignored only in this case (e.g. you're testing that certain configuration leads to a warning), then only put it in the test :)