Add customisable test sizes

We have some tests that we would like to give more resources (memory and time) than the largest size (enormous) that bazel provides. We are currently getting around this by using `bazel run` on the test target. However it would be better if we could provide some custom conditions for a test or even just ignore the constraints for a specific test.

Start by tracing how Bazel assigns resource and time constraints to a test target, then compare that path with the issue's current bazel run workaround. The issue names no source files or tests, so first identify the relevant implementation and test entry points. Done means a documented, tested way to customize or bypass the largest test size constraints.

Written by the indexing model from the issue text.