[dv/dvsim] Add "testfile" grading option
[dv/dvsim] Add "testfile" grading option

Add "testfile" for the coverage reports to contain the list of
tests that contribute to coverage. This enables a better way
to determine the number of reseeds per test.

Signed-off-by: Guillermo Maturana <085716c7110d3f31793f7d964c2e5fd26da1c550@google.com>

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