feat(plugin): wait for initial bundle

This commit adds a specific preprocessor for the all files bundle that
builds the bundle once and passes through the contents later.

The rebuild will be triggered once the bundle itself updates, e.g. via
on('update').

This makes sure the test suite is run ONLY once initially and ONLY after
the bundle itself has been built.

Consecutive changes in test files and or dependencies will lead to a
automatic rebuild.