test(05-04): add failing tests for attach File, Thumb, and bucket

- ThumbFilename/PartitionDirectory/File.Thumb behavior tests
- OpenBucket fail-loud and memblob tests
- Migrate empty plugin list must create system_files

lagoon/attach/migrations.go

@@ -0,0 +1,7 @@
+package attach
+
+import "github.com/go-gormigrate/gormigrate/v2"
+
+// Migrations is the framework-owned system_files set. lagoon.Migrate
+// runs it before every plugin set (D-14).
+var Migrations = []*gormigrate.Migration{}