Followup 19ab72c, add unit test

Adds a unit test to ensure that all providers ignore invalid
ids when passed a FilterFids request.

Spoiler: they do

tests/src/python/providertestbase.py

@@ -383,6 +383,11 @@ def testGetFeaturesFidsTests(self):
         expected = set([fids[1], fids[3], fids[4]])
         assert result == expected, 'Expected {} and got {} when testing for feature IDs filter'.format(expected, result)
 
+        #providers should ignore non-existant fids
+        result = set([f.id() for f in self.provider.getFeatures(QgsFeatureRequest().setFilterFids([-101, fids[1], -102, fids[3], -103, fids[4], -104]))])
+        expected = set([fids[1], fids[3], fids[4]])
+        assert result == expected, 'Expected {} and got {} when testing for feature IDs filter'.format(expected, result)
+
         result = set([f.id() for f in self.provider.getFeatures(QgsFeatureRequest().setFilterFids([]))])
         expected = set([])
         assert result == expected, 'Expected {} and got {} when testing for feature IDs filter'.format(expected, result)