Only insert sources into the fileMap if not already there

This allows us to setup the fileMap correctly for testing so
that it never tries to fetch assets from the CDN.

modules/services/nsi.js

@@ -59,7 +59,7 @@ function setNsiSources() {
 
   let fileMap = fileFetcher.fileMap();
   for (const k in sources) {
-    fileMap[k] = sources[k];
+    if (!fileMap[k]) fileMap[k] = sources[k];
   }
 }