tests: fix race condition in enumeration tests
This change is relevant to Nyquist frequency. To ensure that enumeration has been
run we need to wait at least twice the enumeration timeout. In other words, we need
to make sure enumeration is run at least twice the frequency of our assertions to
ensure that it has been run at least once.
Patch was amended by Alexey Tikhonov <atikhono@redhat.com> to include nice
comment originally provided by Pavel Březina at
https://github.com/SSSD/sssd/pull/947#issuecomment-559440211
Relates: https://pagure.io/SSSD/sssd/issue/3463
Reviewed-by: Michal Židek <mzidek@redhat.com>
Reviewed-by: Alexey Tikhonov <atikhono@redhat.com>