[test] rstmgr compare funcs now check buffer size.
[test] rstmgr compare funcs now check buffer size.

The `rstmgr_testutils_compare_{alert,cpu}_info` functions now check that
the observed buffer size is equal to the actual buffer size.

Signed-off-by: Hugo McNally <97bfee0130c205cc6380afe0c2fb3f741cf89f1f@gmail.com>

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