The `extra_runtime_dependencies` option allows one to activate gems in
the puma master process after "pruning" the master process with
`prune_bundler`. This is useful for activating gems that need to be
loaded in the master process, such as `puma_worker_killer`.

The integration test for `extra_runtime_dependencies` tested the
`$LOAD_PATH` of the worker process instead. Since workers are forked
from the master, it's normally fine to do this, but we might as well
test the master process's `$LOAD_PATH` directly if we can.