d/t/boot-and-services: don't check for first kernel msg if some were missed

If some kernel messages were missed (detected by looking for the 'Missed.*kernel messages' message) then we shouldn't check for the first(ish) kernel message that outputs the 'Command line:', because we likely missed it.

This isn't the fault of systemd/journald, the problem is the kernel filled up its klog buffer before we started journald to read them. This can be caused by a too-small kernel klog buffer, or could be caused by a large number of kernel boot-time messages. More details are in this Ubuntu bug: https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1830479

While catching all the kernel log messages since boot is important, this test case is testing systemd, not the kernel, and should not fail if the kernel's log buffer is filling during boot.

This problem has caused upstream systemd to disable Ubuntu CI on arm64 for many months: https://github.com/systemd/systemd/issues/11104

Closes: #929730