testing/pinta: new aport

Fixes #13337 (closed).

Currently seems to fail on the packaging step; I tried to do some debugging, and it seems to work fine when I run make install manually in the command line, but mysteriously fails in the package step despite printing no visible errors. The return code for all dotnet commands involved in the build in general seems to be 1; I tried calling them manually but to no avail. Just ignoring the exit code isn't an option, because the failed build commands cause the right files to not be installed. Fixed by forcing single-core build, see APKBUILD.