test: Fix getblockstats test data generator

The getblockstats RPC functional test is using previously generated test data that is part of the repository. That test data can be regenerated by running the test file with `--gen-test-data` which invokes the `generate_test_data()` function. That function still relied on the old wallet behavior of having a default wallet to work. Because of this the function was broken and this change fixes this. The fact that this was broken did was not noticed previously because the function is not used by the automated test suite by default.

Author: fjahr

test/functional/rpc_getblockstats.py

@@ -43,6 +43,10 @@ def get_stats(self):
     def generate_test_data(self, filename):
         mocktime = 1525107225
         self.nodes[0].setmocktime(mocktime)
+        self.nodes[0].createwallet(wallet_name='test')
+        privkey = self.nodes[0].get_deterministic_priv_key().key
+        self.nodes[0].importprivkey(privkey)
+
         self.generate(self.nodes[0], COINBASE_MATURITY + 1)
 
         address = self.nodes[0].get_deterministic_priv_key().address