Test null safety modes in modular kernel worker builds

[nshahan]

utils/bazel/kernel_worker.dart requires knowledge of the null safety modes when initializing the frontend compiler. It's not clear if we should be passing flags regarding null safety when creating Targets. This leaves room for bugs, we should add a modular test(s) of some kind that ensures we have updated kernel_worker.dart to correctly support the legacy, weak, and sound modes.

It's possible we could create a modular test suite to run the kernel worker script.

cc @jakemac53