Test relative JSON Pointer index manipulation

[dev-willbird1936]

Summary

• add valid Relative JSON Pointer format cases for forward, backward, and zero index manipulation;
• add malformed index-manipulation controls; and
• cover both Draft 2020-12 and the future v1 draft while leaving older drafts unchanged.

Why

Draft 2020-12 added the optional +N / -N index-manipulation production, but the shared format suite did not exercise it. These cases provide implementation-neutral coverage and are a prerequisite for python-jsonschema/jsonschema#1534.

Specification: https://json-schema.org/draft/2020-12/relative-json-pointer

Validation

• python -m json.tool for both changed files
• python bin/jsonschema_suite check — 11 checks passed, 4 skipped
• git diff --check