Fix Contest Module Test

[mnseok]

Describe the problem and solution

client/contest module에 있는 service test code 들을 client/group 에 있는 test 코드와 같은 형태로 수정합니다.

Validations

• [X] Follow our Code of Conduct
• [X] Read the Contributing Guidelines
• [X] Check that there isn't already an issue