ztest: remove init_testing

When KERNEL was not defined (unittest), the call to init_testing was
used to set a longjump target using 'stack_fail'. When triggered,
this was actually causing a segfault, because longjmp is only valid
if going directly up the stack. Since init_testing returned, it was
no longer on the stack. Instead, that logic MUST be inlined.

Signed-off-by: Yuval Peress <peress@google.com>