Rollup merge of #154504 - cyrgani:structenum, r=Kivooeo

move many tests from `structs-enums` to `structs` or `enum`

This PR moves most of the tests in `ui/structs-enums` that are only about structs or only about enums to their respective directory, as a step towards removing `ui/structs-enums`.
Followup to rust-lang/rust#154131.
r? @Kivooeo

Author: GuillaumeGomez