[tests] added coverage support

tests/.babelrc

@@ -1,5 +1,16 @@
 {
   "presets": [
     "react-native"
-  ]
+  ],
+  "env": {
+    "development": {
+      "plugins": [
+        ["istanbul", {
+          "include": [
+            "**/firebase/**.js"
+          ]
+        }]
+      ]
+    }
+  }
 }

tests/src/tests/index.js

@@ -10,6 +10,10 @@ import config from './config/index';
 import performance from './perf/index';
 import admob from './admob/index';
 
+window.getCoverage = function getCoverage() {
+  return (JSON.stringify(global.__coverage__));
+};
+
 const testSuiteInstances = [
   database,
   auth,