test: shrink `ctor_many_fields`

The test took 90s to 210s in CI and exceeded the 1500s timeout of the sanitizer build. The 900-field structure and 700-field inductive type are replaced by sizes just past the second spill object, and cases that only repeated a layout already covered are dropped.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2 files changed