Es6 nodejs tests

[MarcelloPerathoner]

Update NodeJS tests

• Incorporate eslint suggestions: arrow operator, single quotes, indent=2, etc.
• Load OSRM once only
• Add an environment variable to load OSRM from a configurable directory.

The last point enables testing from a cmake --install directory, so as to catch missing dynamic libraries, wrong RPATH etc.

[DennisOSRM]

Is there a chance we could separate this into two prs, one for reformatting and another for the functional change?

[MarcelloPerathoner]

The functional change is contained entirely in the file constants.js and in the first few lines of each other file, those that say import. The rest is formatting only.

[DennisOSRM]

Could we do this in two PRs? The first changes formatting and the second the functional change.

I know this is a bit cumbersome, but provides more clarity during debugging.