lavamoat-core: modularize test utils

[boneskull]

Various packages (lavamoat-node, lavamoat-browserify, @lavamoat/endomoat) depend on test utils in lavamoat-core. Most/all of the functionality reused involves execution of scenarios.

This necessitates lavamoat-core shipping (and likely generating types for) these utils. This isn't terrible, but it's also not ideal.

Consider yanking stuff like test/util.js out of lavamoat-core and into its own module, e.g., @lavamoat/test-util. Other reusable test utilities could be moved there as well.