Better error message when test code has TLA for CJS distribution

See #110. The error message should say if it is in test code.

Also, perhaps it would be useful to allow skipping running tests for CJS code? Just an idea that probably isn't worth the complexity.

```ts
{
test: "esm",
}
```

Start by reading issue #110 and tracing how the `test: "esm"` configuration is handled when producing the CJS distribution. Confirm the current error path and identify whether it can distinguish test code; done means the error message explicitly says when the problem is in test code. The suggestion to skip CJS tests is an optional follow-up, not a defined requirement.

Written by the indexing model from the issue text.