Assertions are disabled for Verilator

[thomas6785]

prim_assert.sv (from Mocha/OpenTitan) defines the assertion macros, but it disables them for Verilator. Older versions of Verilator did not support assertions but the newest does so they should be enabled to improve checking in tests.

I have run the test suite in my local workspace with assertions 'manually' hacked to verify that they do not fail, but it would be best if we could do this in CI. This would require either:

• Modifying prim_assert.sv in the upstream
• Creating a flow for 'patching' dependencies (similar to the vendor.py flow used in Mocha/OT), but this is somewhat incompatible with using fusesoc.conf