Rollup merge of #86297 - GuillaumeGomez:rustdoc-gui-args, r=Mark-Simulacrum

Allow to pass arguments to rustdoc-gui tool

Very convenient for testing. This is another part of https://github.com/rust-lang/rust/pull/86293

cc ``@jsha``
r? ``@Mark-Simulacrum``

src/bootstrap/test.rs

@@ -894,6 +894,9 @@ impl Step for RustdocGUI {
                 }
             }
         }
+        for test_arg in builder.config.cmd.test_args() {
+            command.arg(test_arg);
+        }
         builder.run(&mut command);
     }
 }