However, just now I'm on a short time-frame to find some open-source, easy-to-use quiz software that we can use to develop a series of tests for assessing numeracy and mathematical skills at higher-education level....

A few robust ones: The AiM Assessment in Mathematics: http://maths.york.ac.uk/aiminfo/ Internet Mathematics Assessment System http://sourceforge.net/projects/imathas/ -Tomas