Fix a bunch of code-checker issues.

backup/moodle2/backup_qtype_stack_plugin.class.php

backup/moodle2/restore_qtype_stack_plugin.class.php

plot.php

question.php

questiontestform.php

questiontestrun.php

renderer.php

stack/answertest/controller.class.php

studentinputs.php

tests/helper.php

tests/test_base.php

tests/walkthrough_adaptive_test.php

tests/walkthrough_deferred_cbm_test.php

tests/walkthrough_deferred_feedback_test.php

tests/walkthrough_immediate_feedback_test.php

tests/walkthrough_interactive_test.php