During the "check flushtree clears adb correctly" check, expecting
"ns.flushtest.example" to always be the first name in the ADB dump is
fragile, because in a certain corner case "a.root-servers.nil" will be
the first name instead.
As the purpose of the relevant check is to ensure "ns.flushtest.example"
is removed from ADB by "rndc flushtree flushtest.example", search the
entire list of names present in ADB instead of just the first entry when
looking for "ns.flushtest.example".
(cherry picked from commit ca1049b2f3)
(cherry picked from commit 3513a585e3)
598d5538fe