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.

js/server_functions.js

@@ -109,6 +109,7 @@ function geExpectedReceivedHeaders (url) {
 function getHtml (req, res) {
 	let html = fs.readFileSync(path.resolve(`${global.root_path}/index.html`), { encoding: "utf8" });
 	html = html.replace("#VERSION#", global.version);
+	html = html.replace("#TESTMODE#", global.mmTestMode);
 
 	let configFile = "config/config.js";
 	if (typeof global.configuration_file !== "undefined") {