Adjust test_concurrency failure modes
Adjust test_concurrency failure modes

The test added for #4686 can raise for "B" missing which
is normal and should not fail the test.  Also ensure mappers are
cleared to prevent subsequent tests elsewhere from being
affected.

Change-Id: I4c5791223e7fd21e04dcd095daa7d868e77dbd97

wujuguang/sqlalchemy,j5int/sqlalchemy,graingert/sqlalchemy,monetate/sqlalchemy,zzzeek/sqlalchemy,j5int/sqlalchemy,monetate/sqlalchemy,graingert/sqlalchemy,sqlalchemy/sqlalchemy,wujuguang/sqlalchemy