tests/btree1: Add testcase for iterating btree object directly.

tests/extmod/btree1.py

@@ -54,3 +54,6 @@ for k, v in db.items(None, None, btree.DESC):
 
 print(list(db.keys()))
 print(list(db.values()))
+
+for k in db:
+    print(k)

tests/extmod/btree1.py.exp

@@ -27,3 +27,6 @@ KeyError
 (b'bar1', b'foo1')
 [b'bar1', b'foo1', b'foo3']
 [b'foo1', b'bar1', b'bar3']
+b'bar1'
+b'foo1'
+b'foo3'