test: check_init_priorities: add a test for initlevels

Add a test for check_init_priorities --initlevels in addition to the
normal error output, this validates that the function names are printed
correctly.

The current case covers sys_init, device with no init functions and
device with init function so it should be all cases.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>