为此，我将定义一个扩展方法来进行正确的Assert：

public static class TestUtils
{
    public static void AssertContainsXTimes(this string actual, string expectedSubstring, int times)
    {
        var actualRemovedLength = actual.Length - actual.Replace(expectedSubstring, string.Empty).Length;
        var expectedRemovedLength = expectedSubstring.Length * times;

        Assert.Equal(expectedRemovedLength, actualRemovedLength);
    }

    public static void AssertContainsAtLeastXTimes(this string actual, string expectedSubstring, int times)
    {
        var actualRemovedLength = actual.Length - actual.Replace(expectedSubstring, string.Empty).Length;
        var expectedRemovedLength = expectedSubstring.Length * times;

        Assert.True(actualRemovedLength >= expectedRemovedLength);
    }
}

下面是示例单元测试的屏幕截图及其结果，以便更好地理解：