Chore: port integration tests to minitest

[casperisfine]

The current integration tests are inherited from Unicorn. They're very hard to debug, especially if they fail on CI.

It would be good to port them to minitest, with Ruby instead of Bash, it would be much easier to produce readable and actionable error reports.

• t0000-http-basic
• t0002-parser-error
• t0004-heartbeat-timeout
• t0009-broken-app
• t0010-reap-logging
• t0013-rewindable-input-false
• t0014-rewindable-input-true
• t0015-configurator-internals
• t0018-write-on-close
• t0019-max_header_len
• t0020-at_exit-handler
• t0022-listener_names-preload_app
• t0100-rack-input-tests
• t0116-client_body_buffer_size
• t0300-no-default-middleware
• t2000-promote-worker
• t9000-preread-input