Run mocha only if only vanilla js tests

[peripateticus]

Currently all tests require webpack build and jsdom. We'd like to run mocha only if you are only testing vanilla js.

Here are example test scripts of the capability we want to capture:

"test-coverage": "vunit --coverage --spec=\"./src/test/js/**/*.?(vue-)spec.js\"",
    "test-js": "cross-env BABEL_ENV=test mocha --require @babel/register --recursive src/test/js/**/*.spec.js",
    "test-js-coverage": "cross-env BABEL_ENV=test nyc mocha --require @babel/register --recursive src/test/js/**/*.spec.js",
    "test-js-coverage-report": "cross-env NODE_ENV=test nyc --reporter=lcov mocha --recursive src/test/js/**/*.spec.js",
    "test-js-watch": "cross-env BABEL_ENV=test mocha --require @babel/register --watch --recursive src/test/js/**/*.spec.js",