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
linux amd64
🐞:sid:amd64

00:00:40

passed #173670
linux amd64
misc:sid:amd64

00:00:20

 
  Build
passed #173682
linux amd64
asan:sid:amd64

00:01:35

canceled #173683
linux amd64
clang:stretch:amd64

00:01:35

passed #173684
linux i386
clang:stretch:i386

00:01:20

passed #173672
linux amd64
docs:sid:amd64

00:00:51

passed #173681
linux amd64
gcc:bionic:amd64

00:01:30

canceled #173673
linux amd64
gcc:centos6:amd64

00:01:37

canceled #173674
linux amd64
gcc:centos7:amd64

00:01:36

canceled #173679
linux amd64
gcc:fedora29:amd64

00:01:39

canceled #173675
linux amd64
gcc:jessie:amd64

00:01:37

canceled #173677
linux amd64
gcc:sid:amd64

00:01:38

passed #173678
linux i386
gcc:sid:i386

00:01:20

passed #173676
linux amd64
gcc:stretch:amd64

00:01:11

passed #173680
linux amd64
gcc:xenial:amd64

00:01:22

passed #173685
linux amd64
pkcs11:sid:amd64

00:01:30

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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