add timing test for many .SD cols

[mattdowle]

• Follow up to streamline loop in GForce j optimization #3777. Add a test in CI somewhere to ensure it doesn't come back. I almost added it to the main test suite but didn't since we need a timing test suite anyway, and the main test suite running on CRAN should be correctness only and quick.

• also follow up on the root cause and make a reproducible example for r-devel. See "Next step would be ..." at the bottom of this comment : streamline loop in GForce j optimization #3777 (comment)