tests: fix expected output when task is specified from Procfile

When run from a Procfile, we output an additional message like "-----> Found 'task' in Procfile, running that command".

tests/unit/builder-dockerfile.bats

@@ -162,7 +162,7 @@ EOF
   echo "output: $output"
   echo "status: $status"
   assert_success
-  assert_output "['task.py', 'test']"
+  assert_output_contains "['task.py', 'test']"
 
   run /bin/bash -c "dokku run $TEST_APP env"
   echo "output: $output"