Delimited text bug fixes 2 - address bug #7688

This patch addresses issue #7688 - QgsFeatureRequest filter type FilterFid not being respected in delimited text provider.  Also addresses a potential core dump from QgsVectorLayerFeatureIterator if invalid attributes requested from in QgsFeatureRequest.setAttributesSubset, identified by new test cases for delimited text provider.

Contains:
Fix to QgsDelimitedTextFeatureIterator
Fix to QgsVectorLayerFeatureIterator
Test cases for QgsFeatureRequest options on delimited text provider.