test: split robustness-spec into module-focused spec files (#417)

The robustness-spec.ts file had grown to 1100+ lines covering tests
across multiple modules. Split it into per-module spec files so tests
live next to the other specs for that module:

- asar-spec.ts: empty/degenerate inputs, stream packaging, roundtrips
- crawlfs-spec.ts: file crawling and type detection
- disk-spec.ts: header reading and filesystem caching
- integrity-spec.ts: hash/block integrity checks
- filesystem-spec.ts: added tree operations (statFile, listPackage)
- pickle-spec.ts: added encoding/decoding edge cases

Also extract the shared tmpdir/fixture helpers into test/util/tmpDir.ts
so each suite gets an isolated per-run temp directory with afterAll
cleanup, instead of duplicating that setup in every file.