test: cluster: remove bind-twice-v1 test

It tests the same thing as bind-twice-v2, only not as in-depth.
test: cluster: rename bind-twice-v2 to bind-twice
test: cluster: unhide child errors in bind-twice

Errors in leaf child processes weren't picked up by the test runner
because they didn't get bubbled up to the main process. Don't forcibly
kill the child processes; tell them to quit gracefully, then inspect
their exit codes.