Rollup merge of #61024 - petrochenkov:proctest, r=nikomatsakis

tests: Centralize proc macros commonly used for testing

Many proc macros in `ui\proc-macro\auxiliary` were doing same things.
(I added a fair share of those myself.)

Now commonly used macros (empty, identity, etc) are collected in one place - `ui\proc-macro\auxiliary\test-macros.rs`.