ses: more coverage for importNow / import

[boneskull]

Given #2804, I feel like we need more coverage for Compartment.prototype.importNow.

• [ ] replicate tests using Compartment.prototype.import and use Compartment.prototype.importNow instead (where feasible; this may include both unitlike & integrationish tests)
• [ ] ensure both CJS and ESM get covered
• [ ] understand what await import() in ESM is doing to avoid redundant tests
• [ ] create failing tests for await import() in CJS (which may be tangential, but I want it anyway)