todo test for GH 20491 (defer inside do block is broken)

This commit adds a todo test for GH 20491. It tests that a defer block
in a single-expression do block runs when exiting said do block,
instead of the block holding the do block.

Author: 1-1sam

t/run/todo.t

@@ -351,6 +351,16 @@ TODO: {
     isnt($?, 0, 'Compilation fails; GH 19378');
 }
 
+TODO: {
+    local $::TODO = 'GH 20491';
+    use experimental 'defer';
+    my $deferred = 0;
+    do {
+        defer { $deferred = 1 };
+    };
+    is($deferred, 1, 'defer in single-expression do block runs when exiting block; GH 20491');
+}
+
 TODO: {
     todo_skip 1 if is_miniperl();
     local $::TODO = 'GH 22168';