Clear failure_log by assignment like as appending by assignment

Author: nobu

bootstraptest/runner.rb

@@ -412,7 +412,7 @@ def show_progress(message = '')
           }
         )
         @@duration = 0
-        @@failure_log.clear
+        @@failure_log = ''
       end
       @@last_test_name = relative_path
       @@duration += t