fix: improve ISA regex validation and add comprehensive test suite

[zouguangxian]

Description

• Fix ISA regex pattern to properly handle sub-extensions like 'RV32I_Zicsr'
• Add comprehensive pytest-based test suite with 100% coverage
• Create developer-friendly Makefile with self-documenting help
• Add TESTING.md with quickstart and usage instructions
• Include test dependencies (pytest, pytest-cov) and configuration
• Add run_tests.py alternative test runner
• Improve regex to handle extension combinations correctly

Related Issues

• isa_regex.match('RV32I_Zicsr') will return None.

Update to/for Ratified/Unratified Extensions

• Ratified
• Unratified

List Extensions

List the extensions that your PR affects. In case of unratified extensions, please provide a link to the spec draft that was referred to make this PR.

Mandatory Checklist:

• Make sure you have updated the versions in setup.cfg and riscv_config/__init__.py. Refer to CONTRIBUTING.rst file for further information.
• Make sure to have created a suitable entry in the CHANGELOG.md.