AST tests: allow matching globals via ast_global
AST: Another failing test: Here nested blocks with global

Probably will turn out to be the same bug as the previously added test,
but it's not exactly the same pattern. For the record, the test added
here:

  ast_contains(sub { our $x; do { do {$x = 123} } },
               ast_block(
                 ast_block(
                   ast_global('$x')
                 )
               ));

Produces this broken AST:

 StatementSequence
   Statement t/037_ast_for.t:89
     While (
         Empty
       StatementSequence <--- FAIL
     ) Continue (
       Empty
     )