test: don't pre-process $KERNEL_APPEND

test: don't pre-process $KERNEL_APPEND

Let's just rely on the word splitting done by bash instead of messing
with that ourselves, as it's just adding extra complexity to appease one
ShellCheck check. Also, this apparently never worked for the nspawn
stuff anyway, since I forgot to set $IFS to an appropriate value, so it
always put all arguments from $KERNEL_APPEND into a single array item
with an extra newline, which then made systemd sad:

~# readarray arr <<< "foo bar baz"; for i in "${arr[@]}"; do echo "'$i'"; done
'foo bar baz
'
~# make -C test/TEST-45-TIMEDATE/ clean setup run BUILD_DIR=$PWD/build TEST_NO_QEMU=1 KERNEL_APPEND="systemd.log_level=console"
...
~# journalctl -o short-monotonic --no-hostname --file /var/tmp/systemd-tests/systemd-test.XaDX67/system.journal --grep "Failed to parse" -p info --no-pager
[551138.986882] systemd-tmpfiles[21]: Failed to parse log level 'console
[551138.987179] systemd-remount-fs[20]: Failed to parse log level 'console
[551138.993125] systemd-sysusers[23]: Failed to parse log level 'console
[551138.998685] journalctl[29]: Failed to parse log level 'console

Resolves: #29945