[sw, dif_plic] Add diagnostic message to one of the test cases
[sw, dif_plic] Add diagnostic message to one of the test cases

The mismatch of in-test register array values with the
RV_PLIC_PARAM_NUMSRC, costed two enginners some debug time.

This change should help to identify the fault quicker.

Signed-off-by: Silvestrs Timofejevs <5a6466a3751cccdb6ee413d1e87698ed8baaba81@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan