Deny warnings in rustdoc non-UI tests

These warnings were silently ignored since they did not appear in a
`.stderr` file and did not fail the test. With this change, warnings in
tests are denied, causing the tests to fail if they have warnings.

I will fix all the warnings that are now test failures next.

src/tools/compiletest/src/runtest.rs

@@ -1452,6 +1452,8 @@ impl<'test> TestCx<'test> {
             .arg(aux_dir)
             .arg("-o")
             .arg(out_dir)
+            .arg("--deny")
+            .arg("warnings")
             .arg(&self.testpaths.file)
             .args(&self.props.compile_flags);