Squashed 'src/leveldb/' changes from a7f9bdc611..13da2d6758

13da2d6758 Merge bitcoin-core/leveldb-subtree#64: refactor: add [[noreturn]] attribute to BackgroundThread*
8c6628bf14 refactor: add [[noreturn]] attribute to BackgroundThread*
8e89537cef Merge bitcoin-core/leveldb-subtree#62: ci: Add LevelDB test workflow
47abe23ff2 ci: run LevelDB tests in GitHub Actions

git-subtree-dir: src/leveldb
git-subtree-split: 13da2d6758396bd21bf76ca76f514700f82b2a62