wait for blackfire agent startup on boot
wait for blackfire agent startup on boot

we're doing the same for e.g. newrelic-daemon

this prevents situations where startup takes a moment, and the app is already sending data

also hardens tests to cover exactly that situation

heroku/heroku-buildpack-php,heroku/heroku-buildpack-php,emeth-/heroku-buildpack-php,emeth-/heroku-buildpack-php,emeth-/heroku-buildpack-php,emeth-/heroku-buildpack-php,heroku/heroku-buildpack-php,heroku/heroku-buildpack-php