Why testkit create TestActor under System Guardian instead of User Guardian?

[yeibon]

We recently updated from 1.1.3 to 1.4.37.

and found that most our test case broken.
that is because TestProbe created under system guardian instead of user guardian.

why changes this? and it seems there is no way to create a TestProbe under user guardian.

because in production code, we use ActorOf to create actor, that also under user guardian.

so we need the TestProbe perform the same way.

any one have any ideas or suggestion how can we fix this problem?