benchmarks: Add a default to the 'file' option of benchmarkTidy.php

benchmarks: Add a default to the 'file' option of benchmarkTidy.php

* Move the fixture to its own directory.
* Add a default to the 'file' option.
* Use loadFile() so that it works by default without requiring
  the user to unzip it first.

Change-Id: I9edf2c19ce5730b72bad3a33c60eda588072a3cf