Load tests from "tests" subdirectory of package directory

[d-torrance]

This update allows package authors to add files to the tests subdirectory of a package directory and have their contents automatically be loaded as package tests (like the Core tests), replacing the need to use TEST get(...). The JSON package is updated to use this feature as a proof of concept.

Surprisingly, only one package (Polyhedra) already had a tests subdirectory, so we shuffle around its files a bit.

Cc: @mikestillman -- we talked about this idea briefly at Fields

[d-torrance]

I think it is a good idea. Just to check: each file is treated as a single test object, and this object is visible with tests("Foo"), right?

Yes, that's exactly right!