Temporarily disable part of PyQgsRulebasedRenderer test

Since testRefineWithCategories and testRefineWithRanges test were
added, this unit test has been crashing on nightlies on rare
occasions.

I can't track down what's causing this, and haven't been able to
reproduce locally.So, I'm going to temporarily skip
testRefineWithRanges to at least try and identify which of the
tests is causing the crash.

tests/src/python/test_qgsrulebasedrenderer.py

@@ -138,6 +138,7 @@ def testRefineWithCategories(self):
         assert self.r3.children()[0].filterExpression() == '"id" = 1'
         assert self.r3.children()[1].filterExpression() == '"id" = 2'
 
+    @unittest.skip("temporarily disabled")
     def testRefineWithRanges(self):
         #Test refining rule with ranges (refs #10815)