ci: x86_64-gnu-tools: Add ad-hoc --test-args regression test

Author: Enselic

src/ci/docker/host-x86_64/x86_64-gnu-tools/Dockerfile

@@ -91,4 +91,9 @@ COPY scripts/shared.sh /scripts/
 
 ENV SCRIPT /tmp/checktools.sh ../x.py && \
   python3 ../x.py check compiletest --set build.compiletest-use-stage0-libtest=true && \
-  python3 ../x.py test tests/rustdoc-gui --stage 2 --test-args "'--jobs 1'"
+  python3 ../x.py test tests/rustdoc-gui --stage 2 --test-args "'--jobs 1'" && \
+  python3 ../x.py test tests/ui --color=never --test-args tests/ui/lint/unused/unused-result.rs --force-rerun | \
+  grep --fixed-strings 'test result: ok. 1 passed; 0 failed; 0 ignored;'
+# The above is a regression test for https://github.com/rust-lang/rust/pull/146501#issuecomment-3292608398.
+# The bug we test for caused 0 tests to run. By grepping on that 1 test was run we prevent
+# regressing. Any single test can be used. We arbitrarily chose `tests/ui/lint/unused/unused-result.rs`.