Improve the way cache contents are searched for "ns.flushtest.example"

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".
2 jobs for 67-stabilize-cacheclean-system-test in 36 minutes and 46 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #1317
linux docker
build:debian:sid:amd64

00:35:54

passed #1318
linux docker
build:debian:sid:i386

00:36:46