[eersnington]

• Makes the backfill resume test prove it resumes: it now fails if the client ignores startAt. Before, it passed either way.
• Removes tests that repeat other tests, and an assertion that could never fail. Merges two overlapping backfill failure tests into one.
• Moves the second-export alarm check into the existing "after migration" test, where it still catches the bug fixed in fix(worker): keep the alarm when an object is exported again #4.
• Removes an unreachable loop condition and return in worker.ts, and fixes a wrong comment about when __migrator is created.
• Shares the snapshot helper between the tests and the smoke script, and type-checks the smoke script.
• Removes unused example options, a redundant tsconfig exclude, a wrong guide snippet, and a stale Miniflare mention.