First off, check your fuse for the starting circuit. If the fuse insn't blown, use a test light to make sure power is getting through that fuse link. Had to replace the fuse link on mine a while back (AFTER I checked out a dozen other things first, of course!). Take a set of jumper cables and ground to the starter or the frame and then hook the positive cable directly to the starter terminal. Bypass the solenoid completely. If the starter turns over normally, you know it's good.  Take a junk screwdriver and cross the small terminal on the solenoid with the big terminal on the solenoid. If you hear a click but the starter doesn't kick in, the solenoid is bad. If nothing at all happens, you've got a bad cable from the battery to the solenoid or bad battery clamps. If you cross the solenoid terminals and the starter turns over normally, then you may have a bad switch or amp meter. 

You mention testing a coil. Is this a gas tractor? If it is, the coil would not have anything to do with the tractor not turning over. Hope this helps out.