Migrate servicetalk-dns-discovery-netty tests to JUnit 5 (#1568)

[kashike]

Motivation:

• JUnit 5 leverages features from Java 8 or later, such as lambda functions, making tests more powerful and easier to maintain.
• JUnit 5 has added some very useful new features for describing, organizing, and executing tests. For instance, tests get better display names and can be organized hierarchically.
• JUnit 5 is organized into multiple libraries, so only the features you need are imported into your project. With build systems such as Maven and Gradle, including the right libraries is easy.
• JUnit 5 can use more than one extension at a time, which JUnit 4 could not (only one runner could be used at a time). This means you can easily combine the Spring extension with other extensions (such as your own custom extension).

Modifications:

• Unit tests have been migrated from JUnit 4 to JUnit 5

Result:

Module servicetalk-dns-discovery-netty now runs tests using JUnit 5