Prevent races when waiting for log messages

The "mirror" system test checks whether log messages announcing a mirror
zone coming into effect are emitted properly.  However, the helper
functions responsible for waiting for zone transfers and zone loading to
complete do not wait for these exact log messages, but rather for other
ones preceding them, which introduces a possibility of false positives.

This problem cannot be addressed by just changing the log message to
look for because the test still needs to discern between transferring a
zone and loading a zone.

Add two new log messages at debug level 99 which are to be emitted after
the log messages announcing a mirror zone coming into effect.  Tweak the
aforementioned helper functions to only return once the log messages
they originally looked for are followed by the newly added log messages.
This reliably prevents races when looking for "mirror zone is now in
use" messages and also enables a workaround previously put into place in
the "mirror" system test to be reverted.
39 jobs from michal/improve-stability-of-mirror-zone-tests in 2 minutes and 20 seconds (queued for 7 seconds)
merge request
Status Job ID Name Coverage
  Precheck
passed #173671
amd64 linux
🐞:sid:amd64

00:00:40

passed #173670
amd64 linux
misc:sid:amd64

00:00:20

 
  Build
passed #173682
amd64 linux
asan:sid:amd64

00:01:35

canceled #173683
amd64 linux
clang:stretch:amd64

00:01:35

passed #173684
i386 linux
clang:stretch:i386

00:01:20

passed #173672
amd64 linux
docs:sid:amd64

00:00:51

passed #173681
amd64 linux
gcc:bionic:amd64

00:01:30

canceled #173673
amd64 linux
gcc:centos6:amd64

00:01:37

canceled #173674
amd64 linux
gcc:centos7:amd64

00:01:36

canceled #173679
amd64 linux
gcc:fedora29:amd64

00:01:39

canceled #173675
amd64 linux
gcc:jessie:amd64

00:01:37

canceled #173677
amd64 linux
gcc:sid:amd64

00:01:38

passed #173678
i386 linux
gcc:sid:i386

00:01:20

passed #173676
amd64 linux
gcc:stretch:amd64

00:01:11

passed #173680
amd64 linux
gcc:xenial:amd64

00:01:22

passed #173685
amd64 linux
pkcs11:sid:amd64

00:01:30

 
  Test
canceled #173704
amd64 linux
system:asan:sid:amd64

canceled #173702
amd64 linux
system:gcc:bionic:amd64

canceled #173686
amd64 linux
system:gcc:centos6:amd64

canceled #173688
amd64 linux
system:gcc:centos7:amd64

canceled #173698
amd64 linux
system:gcc:fedora29:amd64

canceled #173690
amd64 linux
system:gcc:jessie:amd64

canceled #173694
amd64 linux
system:gcc:sid:amd64

canceled #173696
i386 linux
system:gcc:sid:i386

canceled #173692
amd64 linux
system:gcc:stretch:amd64

canceled #173700
amd64 linux
system:gcc:xenial:amd64

canceled #173707
amd64 linux
system:pkcs11:sid:amd64

canceled #173705
amd64 linux
unit:asan:sid:amd64

canceled #173706
amd64 linux
unit:clang:stretch:amd64

canceled #173703
amd64 linux
unit:gcc:bionic:amd64

canceled #173687
amd64 linux
unit:gcc:centos6:amd64

canceled #173689
amd64 linux
unit:gcc:centos7:amd64

canceled #173699
amd64 linux
unit:gcc:fedora29:amd64

canceled #173691
amd64 linux
unit:gcc:jessie:amd64

canceled #173695
amd64 linux
unit:gcc:sid:amd64

canceled #173697
i386 linux
unit:gcc:sid:i386

canceled #173693
amd64 linux
unit:gcc:stretch:amd64

canceled #173701
amd64 linux
unit:gcc:xenial:amd64

canceled #173708
amd64 linux
unit:pkcs11:sid:amd64