[bazel] verilator rebuilds too often

Something is causing the verilated model to rebuild while I'm working on the verilator tests. It's slow and inconvenient. I think the system is just trying not to use all the memory on a workstation and clearing it from the cache.
Until we set up a remote cache it might be worthwhile to use a disk cache by running bazel with a --disk_cache=~/some_cache arg, which is a nice thing to put in a .bazelrc for now.

Sometimes we also appear to build the verilated model for tests that shouldn't depend on it. Because this affects the cw310 tests it's going to impact our CI if we build those tests on the system that runs them.