Fix _check_prompt with interjected logs

[LukasLendvorsky]

Description
ShellDriver._check_prompt uses regexp that expects you get marker immediately followed by prompt, only separated by whitespace.
But sometimes, you can get some unrelated message on the shell console in between the marker and prompt, something like this:

root@user:~# echo 'SVVZ''SYBQPQ'
SVVZSYBQPQ
[   37.369462] systemd-journald[159]: Time jumped backwards, rotating.
root@user:~#

This is enough to break _check_prompt, it will timeout as the regexp never maches. This fixes the issue by waiting separately for the marker and prompt.

[jluebbe]

Nothing guarantees that the kernel log is on its own line. To really fix this, you need to disable console logging via dmesg (which the ShellDriver tries to do very early).