Add uhwm_wait_output cucumber helper

features/steps/output_steps.rb

@@ -1,3 +1,14 @@
+def uhwm_wait_output message, timeout: 1
+  Timeout.timeout(timeout) do
+    loop do
+      break if assert_partial_output_interactive message
+      sleep 0.1
+    end
+  end
+rescue Timeout::Error
+  fail "expected message `#{message}' not seen after #{timeout} seconds"
+end
+
 Then /^the output must contain exactly the usage$/ do
   assert_exact_output <<-eoh, all_output
 Usage: uhwm [options]