Avoid Ruby warning on unused variable

[olleolleolle]

This PR avoids a few Ruby warnings which appeared when I ran the test suite.

Most were in the tests, but one was in a lib/ file.

Example from GitHub Actions:

/Users/runner/runners/2.165.2/work/async-container/async-container/spec/async/container/forked_spec.rb:48: warning: assigned but unused variable - child_pid
/Users/runner/runners/2.165.2/work/async-container/async-container/spec/async/container/notify/pipe_spec.rb:41: warning: assigned but unused variable - child
/Users/runner/runners/2.165.2/work/async-container/async-container/lib/async/container/notify/server.rb:101: warning: assigned but unused variable - address
/Users/runner/runners/2.165.2/work/async-container/async-container/lib/async/container/notify/server.rb:101: warning: assigned but unused variable - flags
/Users/runner/runners/2.165.2/work/async-container/async-container/lib/async/container/notify/server.rb:101: warning: assigned but unused variable - controls

[coveralls]

Coverage remained the same at 79.173% when pulling ca50f8c on olleolleolle:avoid-ruby-warnings-on-unused-var into 45a2f5d on socketry:master.