As the United States seeks to deepen its relationships with African nations and counter the influence of rivals like Russia and China, it confronts a tricky question: How does it respond when countries do things that run afoul of Washington’s stated commitment to democracy and human rights? That tension hung over a major trade conference between the U.S. and African countries that started in Johannesburg this week, after President Biden announced that he was suspending four nations from a critical trade program that aims to promote economic development in Africa.…