tests: Check serial output of firmware boot of some machines

Some of the machines that we have got a firmware image for write
some output to the serial console while booting up. We can use
this output to make sure that the machine is basically working,
so this adds a test that checks the output of these machines
for some well-known "magic" strings.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>