Add testkit API

test/react-native-testkit/lib/comparer.js

   instanceOf : (a, b) => a instanceof b,
   typeof : (a, b) => typeof a === b,
   IsNull : (a, b) => a === null,
-  exists : (a, b) => a !== void 0,
+  exists : (a, b) => a,
   hasValue : (a, b) => (a !== void 0) && (Array.isArray(a) ? a.length !==0 : true),
   isArray : (a, b) => Array.isArray(a),
 }

test/react-native-testkit/lib/test-context.js

           }, this.timeout)
 
           // run test body
-          this.fn.bind(this)(update, data).then((...res) => {
+          new Promise((done) => {
+            this.fn.bind(this)(update, done)
+          })
+          .then((...res) => {
             if(!expired) {
               clearTimeout(tm)
               updateInternal({