rustdoc: Suppress warnings/errors with --test
rustdoc: Suppress warnings/errors with --test

Threads spawned by the test framework have their output captured by default, so
for `rustdoc --test` threads this propagates that capturing to the spawned
thread that we now have.

Closes #39327

GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust