Add a doc_cfg regression test to ensure foreign types impls are working as expected

[GuillaumeGomez]

Fixes #150268.

This is the last case mentioned in the issue that wasn't tested. With this, the issue can be closed.

r? @Urgau

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

............F.............................         (142/142)

======== tests/rustdoc-gui/sidebar.goml ========

[ERROR] line 208: `#sidebar-button a` not found: for command `assert-css: ("#sidebar-button a::before", {"content": |image_url|})`
    at <file:///checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/src/huge_logo/lib.rs.html#2-17>


<= doc-ui tests done: 141 succeeded, 1 failed, 0 filtered out

Error: ()

[GuillaumeGomez]

Oh a flaky. It's been a long time. ^^'