[dv] Directed test to check PoR values of CSRs

[GregAC]

As noted by #2054 our documented mstatus PoR value was incorrect.

We should have a directed test that checks the PoR value of every CSR (or some subset of critical/most interesting CSRs at least) matches the documented value.

[GregAC]

Note that Spike and Ibex seem to disagree on some of the PoR values so some fixes will be needed there to enable this test.