Issue is not reproduced on sqlite backend, which is obviously not a production one. Is reqproduced on the mysql.
I put your code (slightly modified) to LdapIdentitySqlAssignment.

    def test_delete_domain_with_user_added(self):
        domain = {'id': uuid.uuid4().hex, 'name': uuid.uuid4().hex,
                  'enabled': True}
        project = {'id': uuid.uuid4().hex, 'name': uuid.uuid4().hex,
                   'domain_id': domain['id'],
                   'description': uuid.uuid4().hex, 'enabled': True
                   }
        self.assignment_api.create_domain(domain['id'], domain)
        self.assignment_api.create_project(project['id'], project)
        project_ref = self.assignment_api.get_project(project['id'])
        self.assertDictEqual(project_ref, project)

        self.assignment_api.create_grant(user_id=self.user_foo['id'],
                                         project_id=project['id'],
                                         role_id=self.role_member['id'])
        self.assignment_api.delete_grant(user_id=self.user_foo['id'],
                                         project_id=project['id'],
                                         role_id=self.role_member['id'])
        self.assignment_api.delete_domain(domain['id'])
        self.assertRaises(exception.DomainNotFound,
                          self.assignment_api.get_domain,
                          domain['id'])
Test pass ok
But, when I changed config keystone/tests/backend_ldap_sql.conf
#connection = sqlite://
connection = mysql://root:qwerty@127.0.0.1/keystone_test?charset=utf8
Issue was reproduced
nosetests keystone.tests.test_backend_ldap:LdapIdentitySqlAssignment.test_delete_domain_with_user_added
======================================================================
ERROR: keystone.tests.test_backend_ldap.LdapIdentitySqlAssignment.test_delete_domain_with_user_added
----------------------------------------------------------------------
_StringException: traceback-1: {{{
Traceback (most recent call last):
  File "/opt/stack/keystone/keystone/tests/test_backend_ldap.py", line 965, in tearDown
    sql.ModelBase.metadata.drop_all(bind=self.engine)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/schema.py", line 2598, in drop_all
    tables=tables)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 2302, in _run_visitor
    conn._run_visitor(visitorcallable, element, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1972, in _run_visitor
    **kwargs).traverse_single(element)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/sql/visitors.py", line 106, in traverse_single
    return meth(obj, **kw)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/ddl.py", line 130, in visit_metadata
    self.traverse_single(table, drop_ok=True)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/sql/visitors.py", line 106, in traverse_single
    return meth(obj, **kw)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/ddl.py", line 173, in visit_table
    self.connection.execute(schema.DropTable(table))
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1449, in execute
    params)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1542, in _execute_ddl
    compiled
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1698, in _execute_context
    context)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1691, in _execute_context
    context)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/default.py", line 331, in do_execute
    cursor.execute(statement, parameters)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/cursors.py", line 174, in execute
    self.errorhandler(self, exc, value)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorclass, errorvalue
IntegrityError: (IntegrityError) (1217, 'Cannot delete or update a parent row: a foreign key constraint fails') '\nDROP TABLE domain' ()
}}}