case-lib: refactor kernel logs collecting

[ekurdybx]

Refactor kernel messages collecting:

• save journalctl logs from the start of the test, to avoid losing them in unexpected scenarios
• move setup_kernel_check_point function to the start_test func, instead of calling it in every test in different place
• note: some tests setup more kernel checkpoints than one, and check logs for errors every iteration etc. These setup_kernel_check_point calls remain unchanged - the general dmesg.txt since the first checkpoint still will be available though
• in case of kernel checkpoint not being set, we collect all the journalctl logs
• additional: fix shellcheck issues