[SQL] Add average overflow test case from #978

By @egraldlo.

Author: egraldlo <egraldlo@gmail.com>
Author: Michael Armbrust <michael@databricks.com>

Closes #1033 from marmbrus/pr/978 and squashes the following commits:

e228c5e [Michael Armbrust] Remove "test".
762aeaf [Michael Armbrust] Remove unneeded rule. More descriptive name for test table.
d414cd7 [egraldlo] fommatting issues
1153f75 [egraldlo] do best to avoid overflowing in function avg().