[sw, dif_plic] Add a check to DIF PLIC unittests
[sw, dif_plic] Add a check to DIF PLIC unittests

PR #4053 ran into the issue when the unittest interrupt register
offsets were not updated with `RV_PLIC_PARAM_NUMSRC`.

This change aims to make it easier to pinpoint the issue.

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

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