txn_external_test: add test for bounded staleness reads negotiates and fixes the txn timestamp on resume #69386
Description
From @nvanbenschoten (https://github.com/cockroachdb/cockroach/pull/69350#pullrequestreview-738444430)
Do you mind opening an issue for me to add a test to txn_external_test.go that ensures that a bounded staleness read that returns a resume span still negotiates correctly and fixes the transaction timestamp?