kernel: fatal: allow tests to continue upon spurious ISR trigger

We want to be regression-testing the spurious ISR functionality.
Therefore, in z_fatal_error() we need to allow a test to continue
if an error has occured due to a spurious IRQ being triggered.
Only in test mode, wee allow the function to return without an
error. In normal mode the current thread will be aborted.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>