testcases: catch more fatal kernel error messages

Turns out all some messages use lower case, some upper case (for
'fatal'). As well, catch 'Fatal task error!').

Change-Id: I98bb4b00cf2ccee2c87010f4d88cae5f8612ac84
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>