Wait until "rndc dumpdb" completes

"rndc dumpdb" works asynchronously, i.e. the requested dump may not yet
be fully written to disk by the time "rndc" returns.  Prevent false
positives for the "serve-stale" system test by only checking dump
contents after the line indicating that it is complete is written.

(cherry picked from commit 6e3f812a)
39 jobs for !1725 with michal/serve-stale-system-test-wait-for-dump-completion-v9_14 in 12 minutes and 26 seconds (queued for 5 seconds)
detached
Status Job ID Name Coverage
  Precheck
passed 🐞:sid:amd64 #202149
amd64 linux

00:00:41

passed misc:sid:amd64 #202148
amd64 linux

00:00:22

 
  Build
passed asan:sid:amd64 #202160
amd64 linux

00:01:49

passed clang:stretch:amd64 #202161
amd64 linux

00:01:52

passed clang:stretch:i386 #202162
i386 linux

00:02:17

passed docs:sid:amd64 #202150
amd64 linux

00:00:56

passed gcc:bionic:amd64 #202159
amd64 linux

00:01:44

passed gcc:centos6:amd64 #202151
amd64 linux

00:02:18

passed gcc:centos7:amd64 #202152
amd64 linux

00:02:53

passed gcc:fedora29:amd64 #202157
amd64 linux

00:02:32

passed gcc:jessie:amd64 #202153
amd64 linux

00:01:41

passed gcc:sid:amd64 #202155
amd64 linux

00:01:52

passed gcc:sid:i386 #202156
i386 linux

00:01:27

passed gcc:stretch:amd64 #202154
amd64 linux

00:01:32

passed gcc:xenial:amd64 #202158
amd64 linux

00:01:34

passed pkcs11:sid:amd64 #202163
amd64 linux

00:01:47

 
  Test
passed system:asan:sid:amd64 #202182
amd64 linux

00:08:40

passed system:gcc:bionic:amd64 #202180
amd64 linux

00:06:59

passed system:gcc:centos6:amd64 #202164
amd64 linux

00:06:46

passed system:gcc:centos7:amd64 #202166
amd64 linux

00:07:09

passed system:gcc:fedora29:amd64 #202176
amd64 linux

00:07:07

passed system:gcc:jessie:amd64 #202168
amd64 linux

00:07:30

passed system:gcc:sid:amd64 #202172
amd64 linux

00:07:59

passed system:gcc:sid:i386 #202174
i386 linux

00:06:56

passed system:gcc:stretch:amd64 #202170
amd64 linux

00:06:50

passed system:gcc:xenial:amd64 #202178
amd64 linux

00:07:11

passed system:pkcs11:sid:amd64 #202185
amd64 linux

00:07:29

passed unit:asan:sid:amd64 #202183
amd64 linux

00:00:51

passed unit:clang:stretch:amd64 #202184
amd64 linux

00:00:50

passed unit:gcc:bionic:amd64 #202181
amd64 linux

00:00:51

passed unit:gcc:centos6:amd64 #202165
amd64 linux

00:00:49

passed unit:gcc:centos7:amd64 #202167
amd64 linux

00:00:48

passed unit:gcc:fedora29:amd64 #202177
amd64 linux

00:00:53

passed unit:gcc:jessie:amd64 #202169
amd64 linux

00:00:37

passed unit:gcc:sid:amd64 #202173
amd64 linux

00:00:54

passed unit:gcc:sid:i386 #202175
i386 linux

00:00:48

passed unit:gcc:stretch:amd64 #202171
amd64 linux

00:00:55

passed unit:gcc:xenial:amd64 #202179
amd64 linux

00:00:53

passed unit:pkcs11:sid:amd64 #202186
amd64 linux

00:00:46