ApiQuery: Don't mess with PHP output buffering

ApiQuery: Don't mess with PHP output buffering

Specifically, it is not necessary to use output buffering functions
to capture XML generated by the export code because it is already
possible to set the "output sink" object to be used.

* Created a DumpStringOutput class, which appends all output to a
  string property rather than printing output immediately.
* Used that class, instead of ob_start() and ob_get_clean(), in
  ApiQuery and ExportTest.

Change-Id: I238f5d5ec7fd442c845b25cb59ef81ac3285099f