s4:dsdb/test/notification: make test_invalid_filter more resilient against ordering...

s4:dsdb/test/notification: make test_invalid_filter more resilient against ordering races

We saw a lot of flapping tests with:

    [1793(11038)/1892 at 1h55m26s]
    samba4.ldap.notification.python(ad_dc_ntvfs)(ad_dc_ntvfs)
    UNEXPECTED(failure):
    samba4.ldap.notification.python(ad_dc_ntvfs).__main__.LDAPNotificationTest.test_max_search(ad_dc_ntvfs)
    REASON: Exception: Exception: Traceback (most recent call last):
      File
    "/memdisk/autobuild/fl/b1782183/samba/source4/dsdb/tests/python/notification.py",
    line 181, in test_max_search
        self.assertEquals(num, ERR_TIME_LIMIT_EXCEEDED)
    AssertionError: 11 != 3

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>