make test methods static

[Ali-YousefiTelori]

Description

Related issue(s)

Checklist:

• It refers to an Issue.
• It fixes a bug.
• It's a new feature (non-breaking change which adds functionality)
• It's a breaking (fix or feature that would cause existing functionality to change)
• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.
• CHANGELOG is updated.
• Related language label is selected.