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 is what named
instances used in system tests are configured with) that 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" log messages and also enables a workaround
previously put into place in the "mirror" system test to be reverted.
40 jobs for !1505 with michal/improve-stability-of-mirror-zone-tests in 16 minutes and 37 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Precheck
passed #174949
amd64 linux
🐞:sid:amd64

00:00:37

passed #174948
amd64 linux
misc:sid:amd64

00:00:17

 
  Build
passed #174960
amd64 linux
asan:sid:amd64

00:01:27

passed #174961
amd64 linux
clang:stretch:amd64

00:02:06

passed #174962
i386 linux
clang:stretch:i386

00:01:19

passed #174950
amd64 linux
docs:sid:amd64

00:01:17

passed #174959
amd64 linux
gcc:bionic:amd64

00:01:07

passed #174951
amd64 linux
gcc:centos6:amd64

00:02:28

passed #174952
amd64 linux
gcc:centos7:amd64

00:01:57

passed #174957
amd64 linux
gcc:fedora29:amd64

00:02:07

passed #174953
amd64 linux
gcc:jessie:amd64

00:01:03

passed #174955
amd64 linux
gcc:sid:amd64

00:01:32

passed #174956
i386 linux
gcc:sid:i386

00:01:15

passed #174954
amd64 linux
gcc:stretch:amd64

00:01:17

passed #174958
amd64 linux
gcc:xenial:amd64

00:01:45

passed #174963
amd64 linux
pkcs11:sid:amd64

00:01:08

 
  Test
passed #174982
amd64 linux
system:asan:sid:amd64

00:07:47

passed #174980
amd64 linux
system:gcc:bionic:amd64

00:06:57

passed #174964
amd64 linux
system:gcc:centos6:amd64

00:06:11

passed #174966
amd64 linux
system:gcc:centos7:amd64

00:06:33

passed #174976
amd64 linux
system:gcc:fedora29:amd64

00:06:36

passed #174968
amd64 linux
system:gcc:jessie:amd64

00:06:35

passed #174972
amd64 linux
system:gcc:sid:amd64

00:06:40

passed #174974
i386 linux
system:gcc:sid:i386

00:06:32

passed #174987
linux amd64
system:gcc:stretch:amd64

00:06:15

passed #174978
amd64 linux
system:gcc:xenial:amd64

00:06:53

passed #174985
amd64 linux
system:pkcs11:sid:amd64

00:06:50

passed #174983
amd64 linux
unit:asan:sid:amd64

00:01:06

passed #174984
amd64 linux
unit:clang:stretch:amd64

00:00:59

passed #174981
amd64 linux
unit:gcc:bionic:amd64

00:00:54

passed #174965
amd64 linux
unit:gcc:centos6:amd64

00:00:47

passed #174967
amd64 linux
unit:gcc:centos7:amd64

00:00:51

passed #174977
amd64 linux
unit:gcc:fedora29:amd64

00:00:50

passed #174969
amd64 linux
unit:gcc:jessie:amd64

00:00:34

passed #174973
amd64 linux
unit:gcc:sid:amd64

00:00:46

passed #174975
i386 linux
unit:gcc:sid:i386

00:01:00

passed #174971
amd64 linux
unit:gcc:stretch:amd64

00:00:44

passed #174979
amd64 linux
unit:gcc:xenial:amd64

00:01:05

passed #174986
amd64 linux
unit:pkcs11:sid:amd64

00:00:49

failed #174970
amd64 linux
system:gcc:stretch:amd64

00:07:05