Migrate selftests to pytest

[jserv]

This moves selftests into 10 focused pytest modules under 'tests', with shared fixtures and helpers in tests/conftest.py. Original testsuite.py now delegates selftests to pytest and retains only kernel compatibility tests.

• Add pytest.ini with testpaths and compat marker
• Add tests/conftest.py with shared fixtures and assertion helpers (verify_value, assign_and_verify, verify_str, etc.)
• Split inline tests into test_{symbols,repr,expressions,semantics, properties,deps,config_io,lexer,preprocess,compat}
• Update CI workflow to run pytest selftests as a separate step

It also removes testsuite.py in favor of pytest test modules.

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Summary by cubic

Migrated self-tests to pytest with shared fixtures, expanded coverage, and removed deprecated APIs. Compat tests are pytest-marked and run only from a Linux kernel tree; CI and README updated.

• Refactors

  • Split self-tests into 10 pytest modules under tests/ with shared fixtures/helpers in tests/conftest.py; added pytest.ini with a compat marker; compat tests live in tests/test_compat.py.
  • Removed testsuite.py; README updated; tests/reltest now runs only compat tests and example scripts from a kernel tree; CI runs platform-appropriate pytest suites (Windows: headless + menuconfig import only; Linux/macOS: self-tests, compat tests, and examples).
  • Expanded coverage for public APIs: expr_value(), Symbol.config_string, Symbol.rev_dep/weak_rev_dep, Kconfig.missing_syms, and Symbol.user_loc.
  • Inlined load_allconfig into Kconfig.load_allconfig and removed deprecated APIs: KconfigSyntaxError alias, InternalError, and enable_/disable_ warning methods.

• Migration

  • Run self-tests: python -m pytest tests/ -v
  • Skip compat tests: python -m pytest -m "not compat"
  • Run compat tests from a kernel tree: cd /path/to/linux && python -m pytest Kconfiglib/tests/test_compat.py -v

^{Written for commit 4790d62. Summary will update on new commits.}