Reset layers for each pal placement test, avoids all tests failing

because of one

(cherry-picked from 95ecdaf)

Author: nyalldawson

tests/src/python/test_qgspallabeling_placement.py

@@ -54,6 +54,7 @@ def tearDownClass(cls):
     def setUp(self):
         """Run before each test."""
         super(TestPlacementBase, self).setUp()
+        self.removeAllLayers()
         self.configTest('pal_placement', 'sp')
         self._TestImage = ''
         # ensure per test map settings stay encapsulated