Add other compare extensions and tests.

[JeremyKuhne]

Note that xunit bends over backwards when comparing in their asserts. They try to handle nullable, but that should never be an issue as nullable can never satisfy constraints.

[JeremyKuhne]

@dotnet-bot test Portable Windows x64 Release Build

java.lang.NullPointerException?

[danmoseley]

This could be written in just "one" line

        public static void GreaterThanOrEqualTo<T>(T actual, T greaterThanOrEqualTo, string userMessage = null) where T : IComparable
        {
             if ((actual == null && greaterThanOrEqualTo != null) || actual.CompareTo(greaterThanOrEqualTo) < 0)
                throw new XunitException(AddOptionalUserMessage($@"Expected: {actual ?? ""<null>""} to be greater than or equal to {greaterThanOrEqualTo ?? ""<null>""}", userMessage));
        }

[danmoseley]

same for the other one

[JeremyKuhne]

It is a little bit more than that:

            if ((actual == null && greaterThanOrEqualTo != null) || (actual != null && actual.CompareTo(greaterThanOrEqualTo) < 0))
                throw new XunitException(AddOptionalUserMessage($@"Expected: {actual?.ToString() ?? Null} to be greater than or equal to {greaterThanOrEqualTo?.ToString() ?? Null}", userMessage));

I opted for clearer and more verbose in these cases as this is a test assertion. I'm ok with merging the other one (LessThanOrEqualTo), but I'd rather keep the layout the same to make visually comparing the two methods easier.