Migrate Junit 4 to Junit 5

[zhumin8]

similar incentives here: https://github.com/googleapis/sdk-platform-java/issues/1611#issue-1657837566

We should consider modernize our testing infra so we can benefit from new junit 5 features.

In addition, JUnit 5 has an extension pack which allows setting and clearing the env vars: https://junit-pioneer.org/docs/environment-variables/. This could be helpful in simplifying test setup with addition of client logging that depends on env var.