as we are using db_opts for the first time with legacy_schema_args

- fix TablesTest to use the bind that we've returned from setup_bind()
to emit DELETE statements

``legacy_schema_aliasing`` which when set to False will disable a
very old and obsolete behavior, that of the compiler's
attempt to turn all schema-qualified table names into alias names,
to work around old and no longer locatable issues where SQL
server could not parse a multi-part identifier name in all
circumstances.   The behavior prevented more
sophisticated statements from working correctly, including those which
use hints, as well as CRUD statements that embed correlated SELECT
statements.  Rather than continue to repair the feature to work
with more complex statements, it's better to just disable it
as it should no longer be needed for any modern SQL server
version.  The flag defaults to True for the 1.0.x series, leaving
current behavior unchanged for this version series.  In the 1.1
series, it will default to False.  For the 1.0 series,
when not set to either value explicitly, a warning is emitted
when a schema-qualified table is first used in a statement, which
suggests that the flag be set to False for all modern SQL Server
versions.
fixes #3424
fixes #3430

pep-249 exception names linked to exception classes of an entirely
different name, preventing SQLAlchemy's own exception wrapping from
wrapping the error appropriately.
The SQLAlchemy dialect in use needs to implement a new
accessor :attr:`.DefaultDialect.dbapi_exception_translation_map`
to support this feature; this is implemented now for the py-postgresql
dialect.
fixes #3421

The sqlalchemy_exasol dialect needs to support Exasol 4.x which does not support 
the OFFSET feature. Mark test with testing.requires.offset so that they can be skipped
of in the exasol specific test suite.

function patched onto config.  nose/pytest backends now fill
in their exception class here only when loaded
- use more public seeming api to get at py.test Skipped
exception

correctly.
fixes #3406