test: upgrade sniper unit tests to JUnit 5

[gtoison]

Following the discussion on #4780 the new unit test I've added was copied from other existing JUnit 4 tests.
This PR upgrades these similar tests to JUnit 5

[MartinWitt]

LGTM, will merge. Thank you @gtoison

[slarse]

Wait. Wait. @MartinWitt how are we still running JUnit4 tests? Did you not remove those dependencies?

[MartinWitt]

It seems like some dependency is providing JUnit 4 as transitive dependency. And the really strange package import were not matched by my analysis. I never thought anyone creates static package imports.

[gtoison]

I swear I'm innocent!
I looked into it because https://github.com/INRIA/spoon/blob/master/CONTRIBUTING.md says that PR migrating to JUnit 5 are welcome, so maybe this should be amended too?

[slarse]

@MartinWitt From mvn dependency:tree, the venerable junit4.11-beta-1 is pulled in by com.github.stefanbirnker:system-rules.

[INFO] \- com.github.stefanbirkner:system-rules:jar:1.19.0:test
[INFO]    \- junit:junit:jar:4.11-beta-1:test
[INFO]       \- org.hamcrest:hamcrest-core:jar:1.3:test

I don't think any of those rules are used anywhere, so I will attempt to just remove it and try to remove any remaining uses of JUnit4.

@gtoison No worries, it's a good thing we caught on to the fact that we weren't quite as free of JUnit4 as we thought :)

[slarse]

See #4792