Summary:
build_sgd, build_adagrad, and build_adam are in open source python directory
now.
Move the tests to the same directory.
Extract TestBase to test_util.py so that TestFtrl can still refer it.
Depends on D4552227
Reviewed By: salexspb
Differential Revision: D4554549
fbshipit-source-id: 35aed05b82c78530808ef623a25bb7532b2abbae
