Closes https://github.com/go-gitea/gitea/issues/30296
- Adds a DB fixture for actions artifacts
- Adds artifacts test files
- Clears artifacts test files between each run
- Note: I initially initialized the artifacts only for artifacts tests,
but because the files are small it only takes ~8ms, so I changed it to
always run in test setup for simplicity
- Fix some otherwise flaky tests by making them not depend on previous
tests
