Test: Ambigious bindings in same namespace with the same res Add a test based on the discussion [here](https://rust-lang.zulipchat.com/#narrow/channel/421156-gsoc/topic/Project.3A.20Parallel.20Macro.20Expansion/near/542316157) and related to https://github.com/rust-lang/rust/issues/145575#issuecomment-3228868375. This is the most reduced form I could create that passes on nightly but fails with rust-lang/rust#145108 (see [#gsoc > Project: Parallel Macro Expansion @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/421156-gsoc/topic/Project.3A.20Parallel.20Macro.20Expansion/near/542335131)). Also not sure about the test names. r? `@petrochenkov`