Are you positive you wired the starter up correctly?
Last year I had a similar issue after I swapped the gearbox with my father. He unhooked the starter so I asked him to hook it back up. Long story short there are two big bolts and a spade lug. Having 3 wires he plugged in the spade and one of each of the big wires to each bolt. He swore that's how it was before. Car failed the test method above in the car but when I tested it on the bench it worked fine. I thought it might be the relay but it tested fine too. When my battery was completely flat an hour later it all sort of clicked what the problem was.
Charged up the battery, put the two big wires on the outermost bolt/post and she started first go. The inner bolt has a braded copper wire that runs off to the starter motor.
Pretty much what DMAC said in regards to safety. Easiest way to test the starter motor on the bench or in car is a 12v battery and 2 jumper cables. The positive goes to the previously mention bolt/stud/post, the negative ground clamps on the body (usually best on the mounting bolts )
Old screwdriver bridge the spade lug and positive cable. Done it a few times but it always gives me a fright when it suddenly kicks in
