DumpStringOutput: Rename getOutput() to __toString()

DumpStringOutput: Rename getOutput() to __toString()

Though getOutput() is what first came to mind, I do not particularly
like the name, partly because it is used in many, many places as a
method that returns an OutputPage object.

* Renamed the method to __toString(). This is appropriate because
  each instance, at any given time, corresponds to a single string
  value (and exceptions cannot occur during this conversion).
* Removed unnecessary variables from ApiQuery and ExportTest. In
  these and most other cases, it should no longer be necessary to
  call getOutput() explicitly.

Change-Id: Icf202743d1f332f8981338f42eb6e3e5a04abdf1