Separate testing functions into a dedicated testing module

[DevYeom]

Related Issues 💭

Description 📝

• Testing helper functions were separated into a dedicated testing module to prevent build errors during app builds, ensuring they are only available when running tests.

Additional Notes 📚

• When using Testing

@Test
func incrementTwice() async {
    await sut.send(.increment)
    await sut.send(.increment)

    await sut.expect(\.count, 2)
}

• When using XCTest

func test_incrementTwice() async {
    await sut.send(.increment)
    await sut.send(.increment)

    await sut.expect(\.count, 2)
}

Checklist ✅

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