Rollup merge of #114093 - Enselic:stdin-unknown-mod, r=Mark-Simulacrum

Add regression test for `echo 'mod unknown;' | rustc -`

Closes #65601

The bug is fixed since long ago, probably by #69838 (see rust-lang/rust#65601 (comment) for more details).
Add a regression test so we can close the issue.

Author: matthiaskrgr