Big-endian test case fixes: Xoshiro hash

[uweigand]

• Skip endian-dependent part of Xoshiro_AlgorithmBehavesAsExpected on
  big-endian systems

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[uweigand]

@stephentoub Looks like our last two updates crossed each other, sorry. Thanks for the quick Arcade fix! However, if I understand things correctly, this new feature won't be available to "runtime" until there's a new release of Arcade (the Microsoft.DotNet.XUnitExtensions package) and that new release is then pulled into runtime, right?

Should we just wait until that happens, or would my latest version of this patch using the intermediate local property be acceptable to merge as interim solution:

        public static bool IsLittleEndian => BitConverter.IsLittleEndian;
        [ConditionalFact(nameof(IsLittleEndian))]

[danmoseley]

You should be able to just wait for #49820

[uweigand]

This works as expected on my big-endian machine, thanks!