Make shp export use the current working dir from Platform when given a relative path.

This avoids the functional test leaving dangling shape files in the project
folder that then can be committed by mistake.
Fix shp export functional test definition.

The 'Try exporting an inexistent feature type' scenario in ShpExport.feature was working
by accident because previous scenarios were leaving Poinst.shp in the project folder, but
the test definition had the wrong expectation.
Merge remote-tracking branch 'origin/pr/592'