tests: error_hook: fix test_catch_z_oops test case

test_catch_z_oops test case should not run in user mode,
since Z_OOPS needs to be called in supervisor mode, otherwise
we will be getting a reguser user memory access error that is
irrelevant to the test case (and simply is triggered by Z_OOPS
accessing the thread's syscall frame pointer).

In addition to that, we fix the argument of Z_OOPS, because, it
was triggering a null-pointer dereferencing, which results in an
error thrown before Z_OOPS is even executed (if null-pointer
exception is caught).

We also need to generate a dummy thread->syscall_frame, that
Z_OOPS implementation will access. We fix this to
_image_ram_start, because this memory is always part of the
image memory and is always available to supervisor mode. So,
accessing it won't trigger a security fault, for example.

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