Merge specs of UNIXSocket.pair and UNIXSocket.socketpair into a shared spec

[herwinw]

The methods are aliases, so we probably shouldn't have two separate specs hanging around.

[herwinw]

This is a second attempt at #1131, because I've had commit permissions for 1 day and I already screwed things up by merging the wrong branch

[eregon]

I'm confused, this seems to have the same commits as #1131 which is merged (and looks alright and CI passed).

[andrykonchin]

It seems the #1131's commits were removed in the master branch.

[andrykonchin]

Thank you!