community/mptcpd: move from testing

mptcpd is not a new project, but its packages in AlpineLinux are more recent.

I was not sure what I had to do to get it out of testing or simply wait. The wiki is a bit vague about that:

testing: (...) Package will be moved to main or community if there is positive feedback or for other good reasons.

On IRC, @ariadne suggested me to open this MR (thanks!). So here it is.