the truediv test suite didn't have __backend__ so wasn't running
for every DB except in the main build. Repaired this as well
as truediv support to preserve the right-hand side type
when casting to numeric, if the right type is already a
numeric type.
also fixed a memusage test that relies on savepoints so was
not running under gerrit runs.
Change-Id: I3be223fdf697af9c1ed61b70d621f57cbbb7a92b
Mike Bayer
867235e490