contributing: update CI section with current test infrastructure

contributing: update CI section with current test infrastructure

Update the Continuous Integration section to reflect the current state:

- Update the list of runtime-tested architectures (add i386_pentium-mmx
  and mips_24kc, remove outdated i386_pentium4)
- Document the generic test suite (executable, version, hardcoded path,
  strip, linked library, and SONAME checks)
- Add documentation for test-version.sh (version check override) and
  pre-test.sh (pre-test setup) scripts alongside the existing test.sh
- Document available environment variables (PKG_NAME, PKG_VERSION,
  CI_HELPERS) for test scripts

Signed-off-by: Josef Schlehofer <redacted>