Split g_test_log() messages that contain multiple lines

When using TAP we want every single line to be one of the following:

 - a valid TAP clause
 - a comment
 - a blank line

Typical explicit test logs are single line comments, but in some cases
we might end up printing debug messages from libraries, and those may
contain multiple lines. When that happens, we break the TAP and fail the
test in conditions entirely outside of our control.

One option to avoid outright failure is to always prepend each line of a
messge with `#`, to ensure that the whole thing is considered a comment.
10 jobs for backport-2026-log-lines-glib-2-68 in 24 minutes and 59 seconds (queued for 1 second)
Status Job ID Name Coverage
  Style Check
failed #1243066
allowed to fail
check-todos

00:02:27

failed #1243065
allowed to fail
style-check-diff

00:02:26

 
  Build
passed #1243069
cross-android_api21_arm64

00:01:44

passed #1243070
cross-android_api28_arm64

00:01:24

passed #1243071
cross-mingw64

00:02:27

passed #1243068
debian-stable-x86_64

00:02:37

passed #1243067
fedora-x86_64

00:10:39

passed #1243072
win32-ps
msys2-mingw32

00:14:42

passed #1243073
win32-ps
vs2017-x64

00:07:39

 
  Coverage
passed #1243074
coverage

00:03:07

76.3%
 
Name Stage Failure
failed
check-todos Style Check
 * [new tag]               2.66.3                 -> 2.66.3
* [new tag] 2.66.4 -> 2.66.4
From https://gitlab.gnome.org/pwithnall/glib
* branch backport-2026-log-lines-glib-2-68 -> FETCH_HEAD
Couldn’t find common ancestor with upstream master. This typically
happens if you branched from master a long time ago. Please update
your clone, rebase, and re-push your branch.
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
style-check-diff Style Check
 * [new tag]               2.66.3                 -> 2.66.3
* [new tag] 2.66.4 -> 2.66.4
From https://gitlab.gnome.org/pwithnall/glib
* branch backport-2026-log-lines-glib-2-68 -> FETCH_HEAD
Couldn’t find common ancestor with upstream master. This typically
happens if you branched from master a long time ago. Please update
your clone, rebase, and re-push your branch.
Cleaning up file based variables
ERROR: Job failed: exit code 1