Over time, various contributors have asked to be able to be able to access systems to be able to test changes to ports on various buildenvs. Here are our recommendations.
A publicly-accessible, multi-user system called redports is now available. See that page for how to sign up for user accounts.
Port maintainers and committers are also encouraged to install their own Ports Tinderbox for pre-commit testing. (The link contains pointers on how to set it up, and also to the mailing list.) However, most installations only have one buildenv available. (It is possible to cross-build i386 on amd64, however.)