A qmltestrunner abort leaves a `core.<pid>` file beside the tests. The dump
embeds process memory, so it must never reach a commit or a package. Ignore
the kernel's default name so `git add -A` cannot pick one up.

No user-visible behavior changes, so the changelog and the guides stay as they
are; the ignore rule carries its own rationale.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>