A bit more tolerance in provider test

Author: nyalldawson

tests/src/python/providertestbase.py

@@ -876,9 +876,10 @@ def testMinMaxAfterChanges(self):
 
         if vl.dataProvider().capabilities() & QgsVectorDataProvider.DeleteAttributes:
             # delete attributes
-            self.assertTrue(vl.dataProvider().deleteAttributes([0]))
-            self.assertEqual(vl.dataProvider().minimumValue(0), -200)
-            self.assertEqual(vl.dataProvider().maximumValue(0), 400)
+            if vl.dataProvider().deleteAttributes([0]):
+                # may not be possible, e.g. if it's a primary key
+                self.assertEqual(vl.dataProvider().minimumValue(0), -200)
+                self.assertEqual(vl.dataProvider().maximumValue(0), 400)
 
     def testStringComparison(self):
         """