test: use String.prototype.repeat() for clarity

There are a few places where tests repeatedly concatenate strings to
themselves in order to make them very long. Using `.repeat()` makes the
code clearer.

For example, before:

    for (var i = 0; i < 8; ++i) lots_of_headers += lots_of_headers;

After:

    lots_of_headers = lots_of_headers.repeat(256);

Using `.repeat()` makes it clear that the string will be repeated 256
times rather than 8 times. ("What?! That first one doesn't repeat 256
times! It only repeats 8... Oh, wait. Yes, I see your point now.")

PR-URL: https://github.com/nodejs/node/pull/5311


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>