Most of the airborne weather radar sets that I have used (Bendix, RCA mostly) do not radiate in the test mode so can be tested at the gate accordingly.
Also, using the radar on the parallel taxiway approaching the runway is often quite useful as your departure procedure may indeed point you toward that direction. There is absolutely no problem with this procedure because the radiated power output from these sets is generally not dangerous beyond about twenty feet or so.
Have not flown military aircraft so cannot comment about the sets used in these machines.
One middle-east operator that I flew for required the wx radar to be on for takeoff because they believed that birds would be repelled by the radiated energy (even stating that the range should be selected to the lowest possible, for maximum results)...which is complete nonsense with the sets used in commercial aircraft.