Test Suite Migration for Core Utilities

Problem Description

The platform team maintains test suites for two components: a StringFormatter utility and a UserService. Both were written in JUnit 5 some time ago, and the team has been modernising older test files to fit current project conventions.

The files are:

• inputs/StringFormatterTest.kt
• inputs/UserServiceTest.kt

Each file has its own character — read the existing test code to understand how each one is shaped before picking a target style. The team expects the migration to honour the structural intent of each file, not just mechanically port annotations.

Output Specification

Write the converted files to output/StringFormatterTest.kt and output/UserServiceTest.kt.

Every existing test case must be preserved with its intent intact. Any per-test setup logic must continue to run before each test.