Fix bug in "store_fencei_load" test for 40x

Created by: silabs-robin

This PR fixes a bug in the corev_store_fencei_load_instr_stream random test.

The problem was that I used a completely random addresses that were stored to and it led to iss mismatch when certain addresses were used that shouldn't have been used. Fixed by reusing the methods in the parent class riscv_load_store_rand_instr_stream to get sensible addresses.

I have tested with 10 different seeds now and I have not seen any more problems.