SERVER-13635 Added tests for SortedDataInterface.

Expand set of generic SortedDataInterface tests.
Includes tests for the following functions:
  - SortedDataInterface::dupKeyCheck
  - SortedDataInterface::fullValidate
  - SortedDataInterface::getSpaceUsedBytes
  - SortedDataInterface::insert
  - SortedDataInterface::touch
  - SortedDataInterface::unindex
  - SortedDataInterface::Cursor::getDirection
  - SortedDataInterface::Cursor::isEOF
  - SortedDataInterface::Cursor::pointsToSamePlaceAs

Add tests for SortedDataInterface::Cursor::locate.

Add save/restore position tests.
Includes tests for the following functions:
  - SortedDataInterface::Cursor::restorePosition
  - SortedDataInterface::Cursor::savePosition

Add tests for SortedDataInterface::Cursor::advanceTo.
Only tests single-key indices, not those with compound keys.

Closes #804

Signed-off-by: Benety Goh <benety@mongodb.com>