Gate #![reexport_test_harness_main] properly

[Ozzy1423]

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.

[rustbot]

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

[rustbot]

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