Add FAQ entry on testing with a newly installed kernel

Testing Farm does not reboot the test environment after artifact installation, so tests run on the old kernel. Document a tmt prepare step which installs the kernel, reboots via tmt-reboot and skips itself after the reboot. Link the tmt issue tracking native support, https://github.com/teemtee/tmt/issues/5072.

Split out of !292.

Generated-by: Claude Code