Add a test helper function for asynchronous tests

[DevYeom]

Related Issues 💭

Description 📝

• Added a test helper function because the code repeatedly used Task.yield() multiple times for comparing result values.

Additional Notes 📚

// AS-IS
while await sut.state.count < 4 {
    await Task.yield()
}
let state = await sut.state
XCTAssertEqual(state.count, 4)

// TO-DO
await expect { await sut.state.count == 4 }

Checklist ✅

• [x] If it's a new feature, have appropriate unit tests been added?
• [x] If the changes affect existing functionality, please verify whether the above information has been appropriately described.