[dif] Clean up dif_plic_unittests.cc
[dif] Clean up dif_plic_unittests.cc

Signed-off-by: Miguel Young de la Sota <71b8e7f4945fd97b98544cf897992af89646547a@google.com>

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