Explicitly test for Connection in dialect.has_table()

Explicitly test for Connection in dialect.has_table()

The :meth:`_engine.Dialect.has_table` method now raises an informative
exception if a non-Connection is passed to it, as this incorrect behavior
seems to be common.  This method is not intended for external use outside
of a dialect.  Please use the :meth:`.Inspector.has_table` method
or for cross-compatibility with older SQLAlchemy versions, the
:meth:`_engine.Engine.has_table` method.

Fixes: #5780
Fixes: #6062
Fixes: #6260
Change-Id: I9b2439675167019b68d682edee3dcdcfce836987