Reduce conformance test CI time

This limits default architecture set to representative archs (x86_64,
arm64, riscv, arm) instead of iterating all ~24. test_min_config is
further restricted to x86_64 only by default. Full coverage is
preserved via environment variables:
* KCONFIGLIB_OBSESSIVE=1   -- all architectures
* KCONFIGLIB_MIN_CONFIG=1  -- expand test_min_config to representative set

Parametrize test_defconfig, test_min_config, and test_sanity by (arch,
srcarch) so pytest reports per-arch timing and supports selective
re-runs (e.g. -k "test_defconfig[x86_64]").

Also replace shell("cp ...") with shutil.copyfile() in test_min_config
to eliminate shell injection risk from paths with spaces or special
characters.

Author: jserv