Title: #956: tests: fix race confition in enumeration tests
there can be leftovers from previous test run in the passwd result
(enumeration) after the fixture with function scope was executed, but the enumeration
results are still cached in SSSD.
So this first `sleep(INTERACTIVE_TIMEOUT[/2])` is to ensure enumeration task was run and
cleared sysdb cache, right?
But as was explained [here](https://github.com/SSSD/sssd/pull/947#issuecomment-559440211)
test needs to have sleep_timeout > enumeration_timeout to ensure enumeration was run.
I really do not know why we decided to go with INTERACTIVE_TIMEOUT/2
and I wonder if it was supposed to be INTERACTIVE_TIMEOUT*2 instead, which would make much
more sense to me.
See the full comment at https://github.com/SSSD/sssd/pull/956#issuecomment-562209567