Add NewXPendingResult for unit testing XPending

[evilstreak]

I came across this gap whilst writing a unit test. Hopefully this is the right way to add it!

[evilstreak]

Looking at the two failing checks:

• The commit message linter wants a type. I've looked through previous commits to work out the right thing to use, but I get the sense this may only have been introduced recently, as they're mostly typeless. Would this be feat, as it's changing the code to add something new?
• The golang linter is unhappy about the already-existing use of netErr.Temporary in error.go: https://github.com/go-redis/redis/blob/master/error.go#L94. Given that's already existing, I don't think it causes a problem with this commit.

[vmihailenco]

Would this be feat, as it's changing the code to add something new?

Yes, feat looks the right choice here. I will use squah button.

I don't think it causes a problem with this commit

👍

[evilstreak]

Thanks!