Add artifacts test fixture (#30300)

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

modules/storage/storage.go

@@ -131,7 +131,7 @@ var (
 	ActionsArtifacts ObjectStorage = uninitializedStorage
 )
 
-// Init init the stoarge
+// Init init the storage
 func Init() error {
 	for _, f := range []func() error{
 		initAttachments,