Access denied in rust read only database test

[benjaminwinger]

Following up #3283, the rust read only database test is failing to flush the WAL on windows with an access denied error, presumably since the file is open read-only. The reason the similar C API test isn't causing the same problem is that it's disabled on windows (TODO: it should be able to be re-enabled). However SystemConfigTest.testAccessMode should also cover this case on windows and wasn't failing.