removal of usage of global variables

[dionysius]

From MDL-62822

classes/question/bank/question_bank_filter.php
Any reason for using $_POST variables directly on set_defaults()? This is not recommended at all.

classes/question/bank/studentquiz_bank_view.php
Another usages of $_GET and $_POST directly, pretty sure this will require some refactoring.

• removed all usage of $_ params where needed
• during the process detected, that most affected code is not necessary at all, so removed
• category is found by default category of the current context
• From the view.php: if cmid is passed via 'id', 'cmid' is getting set - this place is probably best for that need

This PR needs most likely also a good manual test of the filter! Please have a look into that too.

[dionysius]

Update: fixed unit-test for filtering questions

• The step updateSubmission() in https://github.com/frankkoch/moodle-mod_studentquiz/pull/87/files#diff-6351ea0fef416e5c6111a6f05462ef18L127 was unneccessary, hence moodleform is capable to process the submission by itself. But it wants a valid session, which we have to fake in the unittests.