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/test_framework/test_node.py

@@ -545,6 +545,7 @@ class TestNode():
 
         Will throw if bitcoind starts without an error.
         Will throw if an expected_msg is provided and it does not match bitcoind's stdout."""
+        assert not self.running
         with tempfile.NamedTemporaryFile(dir=self.stderr_dir, delete=False) as log_stderr, \
              tempfile.NamedTemporaryFile(dir=self.stdout_dir, delete=False) as log_stdout:
             try: