Improve Seed#get_key test cases.

The previous version of these tests was using `get_address` just like the code being tested.

It's better to check if the code in question has actually been able to correctly find the public key (which means it also found the correct private key.)

I also added tests that were using other seeds, just in case that particular seed somehow works but others fail.

Because it is doing less tests overall and lower values for `nth`, the new test is also *much* faster.