test: Add build tests

[grypez]

To avoid further hand wringing over vite and vitest we should set up some straightforward post-build tests in proper js. We'll eventually want them anyhow to gate npm releases.

Suggested tests:

• the shims are packaged untransformed
• trusted headers are preserved
• files with trusted headers import them in first position

These tests should run in CI as well as locally. Some of them are perhaps best implemented using shell scripts.

[rekmarks]

This issue is not complete until we also run the build tests in CI.

[rekmarks]

Update: we were running the tests in CI.