docs: add rule testing framework design document

Add the initial design specification for the Minder rule testing
framework. This document proposes a Starlark-based test format with
Go-embedded builtins (eval, read_file, txtar, check) that integrate
with the existing pkg/engine evaluation pipeline.

Key design decisions covered:
- Starlark as the test file format (over Hybrid DSL and txtar+TOML)
- HTTP-level datasource mocking vs abstract box mocking
- Test discovery and file layout options
- Test case declaration approaches (test_*, test() builtin, tuples)
- Assert vs expect assertion semantics
- Standalone ruletest binary (not a minder subcommand for v1)

Related: #6434

Signed-off-by: Krrish Biswas <krrish175@gmail.com>

Author: krrish175-byte