[foozzie] Mock readline for correctness fuzzing

Some test cases stress all global functions and hang when calling
readline.

No-Try: true
Bug: chromium:1034285
Change-Id: I0eaf64437c0806cf3df0a5306aecea4367763cad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1967381
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65460}

tools/clusterfuzz/v8_mock.js

@@ -78,11 +78,12 @@ var prettyPrinted = function prettyPrinted(msg) { return msg; };
   Date = new Proxy(Date, handler);
 })();
 
-// Mock performace methods.
-(function () {
-  performance.now = function () { return 1.2; }
-  performance.measureMemory = function () { return []; }
-})();
+// Mock performance methods.
+performance.now = function () { return 1.2; }
+performance.measureMemory = function () { return []; }
+
+// Mock readline so that test cases don't hang.
+readline = function () { return "foo"; }
 
 // Mock stack traces.
 Error.prepareStackTrace = function (error, structuredStackTrace) {