Add tests for csp.js

Since we lack of tests but got some great point to start, let's write
more tests.

This patch provides some basic tests for our CSP library. It's more an
integration than a unit test, but gets the job done.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

package.json

@@ -184,6 +184,7 @@
     "less-loader": "^4.1.0",
     "mini-css-extract-plugin": "^0.4.1",
     "mocha": "^5.2.0",
+    "mock-require": "^3.0.3",
     "optimize-css-assets-webpack-plugin": "^5.0.0",
     "script-loader": "^0.7.2",
     "string-loader": "^0.0.1",