[tests] Clean up dif_plic_sanitytest.c
[tests] Clean up dif_plic_sanitytest.c

This change makes the PLIC sanity test use a style more consistent with
other on-device tests.

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

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