[Robustness] test observably looks up exec on the object

take 2

lib/results.js

@@ -8,7 +8,7 @@ var resumer = require('resumer');
 var inspect = require('object-inspect');
 var callBound = require('call-bind/callBound');
 var has = require('has');
-var regexpTest = callBound('RegExp.prototype.test');
+var $exec = callBound('RegExp.prototype.exec');
 var yamlIndicators = /:|-|\?/;
 var nextTick = typeof setImmediate !== 'undefined' ? setImmediate : process.nextTick;
 module.exports = Results;
@@ -221,5 +221,5 @@ function getNextTest(results) {
 }
 
 function invalidYaml(str) {
-    return regexpTest(yamlIndicators, str);
+    return $exec(yamlIndicators, str) !== null;
 }