add support for test mode detection in modulename.js via index.html (#3631)

in some cases the modulename.js may need to detect running in test mode
(compliments pr #3630)

window.name is not set  web mode

add a new field to the index.html 
window.intest 
and use the server_function to replace the hard coded string like we do
for window.mmversion=#VERSION#
then change the two  test helpers to set the env variable
app.js detects and sets global.intest=true
server func replace with value of global.intest

then module can use   if(window.intest)

js/app.js

@@ -13,6 +13,7 @@ const { getEnvVarsAsObj } = require(`${__dirname}/server_functions`);
 
 // Get version number.
 global.version = require(`${__dirname}/../package.json`).version;
+global.mmTestMode = process.env.mmTestMode === "true" ? true : false;
 Log.log(`Starting MagicMirror: v${global.version}`);
 
 // Log system information.