Wait for healthy containers in integration test

Rather than waiting 5 seconds and hoping for the best after we boot
docker compose, add docker healthchecks and wait for all the containers
to be healthy.

test/test_helper.rb

@@ -1,6 +1,7 @@
 require "bundler/setup"
 require "active_support/test_case"
 require "active_support/testing/autorun"
+require "active_support/testing/stream"
 require "debug"
 require "mocha/minitest" # using #stubs that can alter returns
 require "minitest/autorun" # using #stub that take args
@@ -23,4 +24,14 @@ module SSHKit
 end
 
 class ActiveSupport::TestCase
+  include ActiveSupport::Testing::Stream
+
+  private
+    def stdouted
+      capture(:stdout) { yield }.strip
+    end
+
+    def stderred
+      capture(:stderr) { yield }.strip
+    end
 end