patch 8.2.4670: memory allocation failures for new tab page not tested

Problem:    Memory allocation failures for new tab page not tested.
Solution:   Add tests with failing memory allocation. (Yegappan Lakshmanan,
            closes #10067)
diff --git a/src/buffer.c b/src/buffer.c
index c8e63c3..8b50540 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2093,7 +2093,7 @@
 	}
 #ifdef FEAT_EVAL
 	// init b: variables
-	buf->b_vars = dict_alloc_id(aid_buflistnew_bvars);
+	buf->b_vars = dict_alloc_id(aid_newbuf_bvars);
 	if (buf->b_vars == NULL)
 	{
 	    vim_free(ffname);