tests: coredump: Call k_panic() directly for mpfs_icicle board

mpfs_icicle timeouts on this test similarly to other boards
simulated in Renode. Null pointer dereference doesn't trigger
a CPU fault during simulation. Instead, call k_panic() directly.

Link: https://github.com/zephyrproject-rtos/zephyr/commit/a28d5df5c643614042ca04caf65fcb274c1e956a



Signed-off-by: Marek Slowinski <mslowinski@antmicro.com>