Rollup merge of #152210 - Ozzy1423:attrs7, r=JonathanBrouwer

Gate #![reexport_test_harness_main] properly

Address the FIXME

Removed from `issue-43106-gating-of-builtin-attrs.rs` since that is for stable attributes only.

This would be a breaking change, search of github shows it is mostly but not always used with `#![test_runner]` which is already gated correctly.

Details:
rust-lang/rust#50297

Feel free to close this issue if you think it is not worth addressing the FIXME...

Author: JonathanBrouwer