Add unit test for stack

- Also, fix bug where trees pushed onto the stack were not retained

spec/runtime/runtime_spec_helper.h

@@ -3,6 +3,7 @@
 
 #include "bandit/bandit.h"
 #include "tree_sitter/runtime.h"
+#include "helpers/tree_helpers.h"
 
 using namespace std;
 using namespace bandit;