Test the NodeJS Component

Previously, the NodeJS component class was untested. This adds test
coverage for it.

[#71846956]
It should fail releases with no start script present

The Node buildpack requires either a server.js script to be present or for a
start script to be specified in the package.json file. The commit will cause
the release phase of the buildpack to fail with a suitable warning message if
this is not the case.

[#71846414]