Disarm drop bombs for unexecuted test Cargo commands

The code for running tests uses a custom command machinery because it streams the output of the command. We thus need to mark the command as executed in a dry run, to avoid a drop bomb panic.

src/bootstrap/src/utils/render_tests.rs

@@ -33,6 +33,7 @@ pub(crate) fn try_run_tests(
     stream: bool,
 ) -> bool {
     if builder.config.dry_run() {
+        cmd.mark_as_executed();
         return true;
     }