[sw, dif_rstmgr] Clarify a check in one of the unittests
[sw, dif_rstmgr] Clarify a check in one of the unittests

In one of the previous Reset Manager changes, it came up that this
check was not obvious.

This follow-up is a part of #3454

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

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