public async Task AddCountry_ShouldNotAdd_WhenCountryExists()
{
// Arrange
var responseMessage = "Country already exists.";
_unitOfWorkMock.Setup(x => x.Countries.Exists(It.IsAny <string>()))
.ReturnsAsync(true);
// Act
var result = await _sut.AddCountry(It.IsAny <string>());
// Assert
Assert.Equal(responseMessage, result.Value);
}