Added test cases to write to CSRs

Created by: dherreravicioso

These test cases add coverage for writing to STVAL, SCAUSE, SEPC, and STIMECMP CSRs. Regression still passes after adding these lines. These additions may not be necessary in the future, as Prof. Harris alluded to there being some weird behavior regarding the test coverage report. For the time being, these tests increase the coverage in the privilege unit.

Thanks!