Split validation tests into 16 topic-based submodules

Break up the monolithic 32,400-line validation/tests/mod.rs into
focused submodules: layout_ordering, module_basics, functions, cfg,
capabilities, arithmetic, composites, access_chain, block_layout,
decorations, builtins, storage_classes, types, versions, entry_points,
and misc. Shared test helpers moved to prelude.rs.

Author: LegNeato