check dmesg overwrites itself with tests that reboot

[sbertramrh]

tmt-reboots dmesg-after will run when the reboot is called. Not an issue in general but the problem occurs when the test completes or has multiple reboots, the name of the file used to save it is the same.
/var/tmp/tmt/run-022/kernel-automotive/plans/sst_kernel_ft/memory/plan/execute/data/guest/default-0/external_er/memory/config_memtest/config_memtest-1/checks/dmesg-after-test.txt
So it will be overwritten each time and only the very last one will remain. I think the same is happening with the dmesg-before-test.txt as well. Can we index this during a single test run so that we have the contents of all the captures?

Can we create the log file capture from dmesg with the epoch date appended in the name, so something like:
dmesg-after-test-$(date +%s).txt?
Or equivalent for python syntax.