Add async expectation helper functions for store state validation

[DevYeom]

Related Issues 💭

Description 📝

• Added two functions to easily validate the state of the store when writing tests.
  • expect(::timeout:sourceLocation:)
  • xctExpect(::timeout:file:line:)
• Updated the documentation related to this.

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.xctExpect(\.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.

[DevYeom]

All have been deprecated and remade in #97.