Upgrade JUnit and Mockito

[wborn]

Upgrades JUnit from 5.7.0 to 5.8.1 Upgrades Mockito from 3.7.0 to 4.0.0

For release notes, see:

• https://junit.org/junit5/docs/current/release-notes/index.html
• https://github.com/mockito/mockito/releases