Use for..of Object.keys instead of for..in (#9518)

In https://github.com/babel/babel/issues/9511 (and #9495 is another symptom), @PavelKastornyy reported a node crash becaue the JavaScript heap run out of memory. The problem was that their code was adding enumerable properties to `Object.prototype`: it is something that shouldn't be done, but Babel shouldn't make node crash if someone adds them.
I reduced down the problem to `for...in` loops in `@babel/traverse` that grew the memory consumption exponentially because of that unexpected properties.

packages/babel-parser/test/helpers/runFixtureTests.js

@@ -195,12 +195,12 @@ function misMatch(exp, act) {
       return ppJSON(exp) + " !== " + ppJSON(act);
     }
   } else {
-    for (const prop in exp) {
+    for (const prop of Object.keys(exp)) {
       const mis = misMatch(exp[prop], act[prop]);
       if (mis) return addPath(mis, prop);
     }
 
-    for (const prop in act) {
+    for (const prop of Object.keys(act)) {
       if (typeof act[prop] === "function") {
         continue;
       }