Here in Minnesota, capping a line can cause leaks, since you would have a section of line that would trap and hold water over the winter, which would freeze and crack the pipe. Your best bet would be to connect the new line, test, and if not to your liking, eliminate the new line at the connection point.
If freezing is of no concern to you, then a 1"x1/2" fpt elbow and a 1/2" mpt cap will make a makeshift plug. I would still advise eliminating at the connection point, however.