Add unit test for Model groupId

[chenchc6]

Description

This pull request adds a new unit test for the Model class in the Apache Maven project. The test verifies the correctness of the Model class's setGroupId and getGroupId methods by asserting the expected values.

Changes Made

• Added ModelTest.java under maven-model/src/test/java/org/apache/maven/model/
• The test case validates:
  • Group ID is correctly assigned and retrieved

Purpose

The purpose of this test is to ensure that the Model class correctly sets and retrieves its groupId field. This helps in maintaining the robustness and reliability of the Apache Maven project.

Contribution

• Verifying the correctness of the Model class's behavior.
• Enhancing the test coverage of the Apache Maven codebase.
• Providing a reference for future tests and improvements.