Only do smoke test during release

This change introduces a new command make smoke-test, and it changes _support/release to only do a smoke test.

The motivation for this is that it's currently too slow to do releases; particularly when back-porting.

The reason we run tests before releasing is that CI only acts as a gatekeeper before merging a MR, and it has happened (and will continue to happen) that code breaks because of the merge. E.g. duplicate Go function definitions.

The smoke test includes compiling the Go executables, running the rspec test suite, and testing the rubyserver go package. This is not the same thing as running the full test suite but it gives a reasonable amount of confidence that the merge didn't break anything.