test: Add header-only test framework

Adds src/test/util/framework.hpp as a lightweight Boost.Test replacement.
This commit only introduces the header; build-system integration and
call-site migration follow in subsequent commits.

Includes:
- `CHECK`, valid with any comparison operator, optional message
- `REQUIRE`, valid with any comparison operator, optional message
- `CHECK_EQUAL_RANGES`, better debugging for vectors
- `THROW_*`, macros for checking throwing conditions
- Info and warn messages

Author: rustaceanrob