繼續填坑，之前從代碼邏輯上走了一圈，沒啥太深印象，看來需要花個圖看看
這個visit_suite貌似比較複雜，代碼裏專門描述了算法：
Visitor algorithm
All suite, test, keyword and message objects have a visit() method
that accepts a visitor instance. These methods will then call the correct visitor method visit_suite(), visit_test(),visit_keyword() or visit_message(),
depending on the instance where the visit() method
exists.
The recommended and definitely easiest way to implement a visitor is extending the SuiteVisitorbase
class. The default implementation of its visit_x() methods
take care of traversing child elements of the object x recursively.
A visit_x() method
first calls a corresponding start_x()method
(e.g. visit_suite() calls start_suite()),
then calls visit() for
all child objects of the xobject,
and finally calls the corresponding end_x() method.
The default implementations of start_x() and end_x() do
nothing.
Visitors extending the SuiteVisitor can
stop visiting at a certain level either by overriding suitable visit_x() method
or by returning an explicit False from
any start_x() method.