linked_attribute tests: helper assert function for expected LdbError

linked_attribute tests: helper assert function for expected LdbError

The logic involved in asserting that a function raises an LdbError with
a particular error value has shown itself to be too complicated for me
to repeat too often.

To test this function, you would want a put a test in a bit like this:

    def test_assertRaisesLdbError(self):
        for i in [1, 2, ldb.ERR_ENTRY_ALREADY_EXISTS, 999]:
            def f(*args, **kwargs):
                raise ldb.LdbError(i, 'msg %s' % i)
            self.assertRaisesLdbError(i, 'a message', f, 'la la', la='la')

            def f2(*args, **kwargs):
                raise ldb.LdbError(i + 1, 'msg %s' % i)
            def f3(*args, **kwargs):
                pass
            for f in (f2, f3):
                try:
                    self.assertRaisesLdbError(i, 'a message', f, 'la la', la='la')
                except AssertionError as e:
                    print i, e, f
                    pass
                else:
                    print i, f
                    self.fail('assertRaisesLdbError() failed to fail!')

..but a self-testing test-tester is getting a too meta to run in every
autobuild.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>