test: stop node before calling assert_start_raises_init_error

...in feature_coinstatsindex and feature_pruning.
Also add an assert to assert_start_raises_init_error that the node is
not already running.

test/functional/feature_pruning.py

@@ -125,6 +125,7 @@ class PruneTest(BitcoinTestFramework):
         self.sync_blocks(self.nodes[0:5])
 
     def test_invalid_command_line_options(self):
+        self.stop_node(0)
         self.nodes[0].assert_start_raises_init_error(
             expected_msg='Error: Prune cannot be configured with a negative value.',
             extra_args=['-prune=-1'],