Move to Junit5 in pekko 2.0?

[He-Pin]

Motivation:
Junit 5 has many new function, eg dynamic testing, and I think we can move to it later.

[mdedetrich]

Are you talking about our internal Java tests or the testkit?

[He-Pin]

@mdedetrich both, it is currently using Junit 4

[pjfanning]

We do have some Junit5 support.
Example:
https://github.com/search?q=repo%3Aapache%2Fpekko%20TestKitJUnit5Extension&type=code

I would prefer if we add missing junit 5 support as opposed to just dropping junit 4. If we can prove that junit 4 support is making supporting junit 5 hard, then let's discuss junit 4 removal then.

If someone wants to rewrite some tests that are using junit 4 and make them use junit 5 then that's possibly useful but not the biggest win in terms of things that we could spend time on.