Fix an issue with small lists

[bradunov]

When a list of all questions applicable to a test is too small, less than a number of questions to be asked in the text, the site fails. One example is:
https://www.tatamata.org/main?op=test&q_id=&l_id=&year=drugi&theme=brojevi&subtheme=brojevni%20izrazi&period=jesen&difficulty=2&language=rs&design=dev
Fix this.

[bradunov]

Changed the list selection algorithm and fixed it.