Smoketests: remove interface for using a module source string; always require a precompiled module

Smoketests which use precompiled modules have better performance characteristics than those that use modules with source strings (mostly because our CI can be smarter about them), so we want to require that all smoketests use precompiled modules. Currently, we have a few smoketests which use non-precompiled modules specifically because they assert that the compilation will fail. We should rewrite those tests so that they either call spacetime build explicitly or use a separate, purpose-built abstraction, rather than calling a normal Smoketest constructor and asserting that it fails. Then, we should remove the Smoketest constructor that accepts a source string, so that all smoketests have to use precompiled modules.

Start by locating the Smoketest constructor that accepts a source string and all smoketests using it. Review the existing compilation-failure cases, then confirm that they use an explicit build or separate abstraction and that every remaining Smoketest uses a precompiled module; the constructor should no longer accept source strings.