Reorganize CubicTest.cpp: group tests by category with banner comments

Group the 50 CUBIC tests into three sections with descriptive banners:
- Specification-conformance tests (23): RFC 8312, RFC 9002, RFC 9406
- Implementation-specific tests - loosely coupled (7)
- Implementation-specific tests - tightly coupled (20)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: saikat107