Add some tests and docs for visibility attributes

[Jane Sandberg]

I spent some time today trying to wrap my mind around visibility attributes used for Search (e.g. the search.calculate_visibility_attribute stored procedure and friends). At the end of it, I wrote some documentation and characterization tests that will hopefully help the next person who digs into this, and of course provide a bit of a safety net for the next person who has to make changes to it. The docs are intended as an in-code complement to the excellent docs/TechRef/PureSQLSearch.adoc.

Branch forthcoming.

[Jane Sandberg (sandbergja)]

Here is the branch: user/sandbergja/visibility-pgtap

Here is a link: https://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/sandbergja/visibility-pgtap

Here is a refresher on how to run pgtap tests: https://wiki.evergreen-ils.org/doku.php?id=dev:contributing:qa#pgtap_database_tests

...and now, back to totally forgetting how bitwise operators work 🤣