[dev] Replace istanbul with nyc

Inizia con il comando grunt test e trova la dipendenza Istanbul e la configurazione della coverage utilizzate dalla CLI. Verifica come la build Jenkins riceve i fallimenti dei test e verifica la migrazione rispetto all’esempio di destructuring che attualmente genera un errore di sintassi. Il lavoro è completato quando i test vengono eseguiti con nyc e gli errori di coverage producono uno stato diverso da zero.

As the istanbul package is not longer maintained, we need to migrate to nyc.
Currently istanbul doesn't work for the following code:

const { stdio = "inherit" } = options;

In this case grunt test command fails with an exception:

__cov_gCRxMpJElFAq6oDSOHYQNQ.s['1']++;var __awaiter=(__cov_gCRxMpJElFAq6oDSOHYQNQ.b['1'][0]++,this)&&(__cov_gCRxMpJElFAq6oDSOHYQNQ.b['1'][1]++,this.__awaiter)||(__cov_gCRxMpJElFAq6oDSOHYQNQ.b['1'][2]++,function(thisArg,_arguments,P,generator){__cov_gCRxMpJElFAq6oDSOHYQNQ.f['1']++;__cov_gCRxMpJElFAq6oDSOHYQNQ.s['2']++;return new((__cov_gCRxMpJElFAq6oDSOHYQNQ.b['2'][0]++,P)||(__cov_gCRxMpJElFAq6oDSOHYQNQ.b['2'][1]++,P=Promise))(function(resolve,reject){__cov_gCRxMpJElFAq6oDSOHYQNQ.f['2']++;function fulfilled(value){__cov_gCRxMpJElFAq6oDSOHYQNQ.f['3']++;__cov_gCRxMpJElFAq6oDSOHYQNQ.s['4']++;try{__cov_gCRxMpJElFAq6oDSOHYQNQ.s['5']++;step(generator.next(value));}catch(e){__cov_gCRxMpJElFAq6oDSOHYQNQ.s['6']++;reject(e);}}function rejected(value){__cov_gCRxMpJElFAq6oDSOHYQNQ.f['4']++;__cov_gCRxMpJElFAq6oDSOHYQNQ.s['8']++;try{__cov_gCRxMpJElFAq6oDSOHYQNQ.s['9']++;step(generator['throw'](value));}catch(e){__cov_gCRxMpJElFAq6oDSO

SyntaxError: Unexpected token :
    at new Script (vm.js:79:7)
    at createScript (vm.js:251:10)
    at Object.runInThisContext (vm.js:303:10)
    at Module._compile (internal/modules/cjs/loader.js:656:28)
    at Object.Module._extensions.(anonymous function) [as .js] (/Users/havaluova/Work3/nativescript-cli/node_modules/istanbul/lib/hook.js:107:24)
    at Module.load (internal/modules/cjs/loader.js:598:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:537:12)
    at Function.Module._load (internal/modules/cjs/loader.js:529:3)
    at Module.require (internal/modules/cjs/loader.js:636:17)
    at require (internal/modules/cjs/helpers.js:20:18)

It seems that when the above mentioned error occurs, istanbul exits with status code 0 and this is the reason why jenkins build doesn't fail in such situation.