Expect ordering NULLs to work and skip nested aggregate tests on sqlite >= 3.30.

Expect ordering NULLs to work and skip nested aggregate tests on sqlite >= 3.30.

Fixed test failures which would occur with newer SQLite as of version 3.30
or greater, due to their addition of nulls ordering syntax as well as new
restrictions on aggregate functions.  Pull request courtesy Nils Philippsen.

Fixes: #4920
Closes: #4921
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4921
Pull-request-sha: e18534a9045786efdaf4963515222838c62e0300

Change-Id: Idb2fc96e23e7cc96cf96f586208c2db561518202