tests: debug/coredump: change to dump minimal memory

The default of coredump is to dump all RAM as defined in
the linker script. However, this takes a while on boards
with bigger memory space, and would slow down automated
testing due to the need to grab all the data via serial.
So change it to dump minimal memory, as this would still
test the memory dumping capability.

Fixes #28547

Signed-off-by: Daniel Leung <daniel.leung@intel.com>