Update environment variable unit tests with Junit 5 features after dropping Java 8 support

[JoeWang1127]

We tried to use Junit 5 extensions to update unit tests that depends on preset environment variables (see #2799).

However, junit-pioneer is not compatible with Java 8 (so as other extensions like system-stubs).

We should update unit tests after dropping Java 8 support. See https://github.com/googleapis/sdk-platform-java/pull/2724#discussion_r1605456611 for details.