Figure out test cases for WITHIN

[scotws]

WITHIN currently has no test cases, and there are none included in the standard (see https://forth-standard.org/standard/core/WITHIN) -- it just states that their implementation : WITHIN ( test low high -- flag ) OVER - >R - R> U< ; "works in all cases". Gforth (mirror at https://github.com/forthy42/gforth/blob/master/test/core.fs) also doesn't seem to test this. The problem is that the word gets tricky for signed and unsigned numbers, and it would be nice to do WITHIN in assembler because the JSR use lots of cycles for simple moves to and from the Return Stack.