sir my question is that why we take positive charge as a test charge??

It doesn''t matters.

U can take  negative charge as a test charge answer will remain same.

APPROVE!

IT IS ONLY FOR OUR CONVENIENCE  WE DO SO OTHERWISE U CAN TAKE NEGATIVE AS A TEST CHARGE OF ANY MAGNITUDE THE ANSWER WILL BE SAME BECAUSE TEST CHARGE GETS CUT OUT FROM THE EXPRESSIONS OR REMAINS SILENT FEATURED...............

IT IS ONLY FOR OUR CONVENIENCE  WE DO SO OTHERWISE U CAN TAKE NEGATIVE AS A TEST CHARGE OF ANY MAGNITUDE THE ANSWER WILL BE SAME BECAUSE TEST CHARGE GETS CUT OUT FROM THE EXPRESSIONS OR REMAINS SILENT FEATURED