More selection unit tests for variables and fixes.

More selection unit tests for variables and fixes.

- Added unit tests for more complex selections where either numeric or
  position values were assigned to variables.
- Fixed handling of subexpressions in these cases (will back-port to
  older branches separately).
- Added unit tests for constant expressions in variables (in particular
  constant positions were not handled right during initial development
  of selections).
- Added comments in the code for some parts that are currently
  unreachable.

Part of #651.

Change-Id: Ifbb585ca00fd7a9715b4c9cb004c3a895551c1a6