Junit5 cleanup / best practices

[onobc]

There are several places in the SCS repo that have older Junit4 usage and can be refreshed to Junit5 usage/best practices:

• remove @ExtendWith(SpringExtension.class) from all @SpringBootTest as SBT includes that annotation.(comment)

• leverage package-protected test classes and test methods (comment)

• {optional): remove 'test' method name prefix

[sobychacko]

These tasks have now been completed. A few tests remain with the test prefix due to some cross-test interaction. We will tackle those separately later.