Configure uhwm_wait_output test helper via the env

The `timeout' keyword argument was never used, replace it with a test
for UHWMTEST_OUTPUT_TIMEOUT environment variable and use the value when
set.

lib/uh/wm/testing/acceptance_helpers.rb

@@ -22,7 +22,10 @@ module Uh
           @process.stdout
         end
 
-        def uhwm_wait_output message, timeout: 1
+        def uhwm_wait_output message
+          timeout = ENV.key?('UHWMTEST_OUTPUT_TIMEOUT') ?
+            ENV['UHWMTEST_OUTPUT_TIMEOUT'].to_i :
+            1
           Timeout.timeout(timeout) do
             loop do
               break if case message