Fix: Test modules allow node_modules to be checked in to git

From the gitignore: 

```
!tests/*/node_modules/
!tests/*/node_modules/*/*/lib/
!tests/*/node_modules/*/dist/
```

And we can see, these directories are checked in to git. That should be not allowed. 

Convert the test modules to private monorepo modules (using yarn workspaces) and remove node_modules from being checked in.