Run unit tests in Deno too

[qgustavor]

At the moment only integration tests are run in Deno (those which use the compiled library), not unit tests (those which load parts of the code from the source).

What is blocking this issue is how to implement it: is there a way to implement it without duplicating a lot of code that handle Node polyfills between build.js and test/helpers/test-runner.mjs?