Set up mo-test

[rvanasa]

This PR configures mo-test (CI runner) and mo-dev --test (file watcher) via npm scripts.

Here is an alternate way to configure the Motoko package sources using the project's local MOPS installation:

dfx.json:

{
  "defaults": {
    "build": {
      "packtool": "npm run --silent sources"
    }
  }
}

package.json:

{
  "scripts": {
    "sources": "mops sources"
  }
}