Merge #18543: test: Use one node to avoid a race due to missing sync in rpc_signrawtransaction

fa2251df5e3471e0d19d5d5c7780d34ee1498cee test: Use one node to avoid a race due to missing sync in rpc_signrawtransaction (MarcoFalke)

Pull request description:

  Node 0 creates a transaction in a block, and node 1 sends a spending transaction without properly syncing the utxo set.

  Fixes intermittent test failure in rpc_signrawtransaction

  ```
  test  2020-04-01T00:14:03.400000Z TestFramework (ERROR): JSONRPC error
                                     Traceback (most recent call last):
                                       File "C:\projects\bitcoin\test\functional\test_framework\test_framework.py", line 112, in main
                                         self.run_test()
                                       File "C:\projects\bitcoin/test/functional/rpc_signrawtransaction.py", line 213, in run_test
                                         self.witness_script_test()
                                       File "C:\projects\bitcoin/test/functional/rpc_signrawtransaction.py", line 208, in witness_script_test
                                         self.nodes[1].sendrawtransaction(spending_tx_signed['hex'])
                                       File "C:\projects\bitcoin\test\functional\test_framework\coverage.py", line 47, in __call__
                                         return_val = self.auth_service_proxy_instance.__call__(*args, **kwargs)
                                       File "C:\projects\bitcoin\test\functional\test_framework\authproxy.py", line 141, in __call__
                                         raise JSONRPCException(response['error'], status)
                                     test_framework.authproxy.JSONRPCException: bad-txns-inputs-missingorspent (-25)
  ```

  Full log: https://ci.appveyor.com/project/DrahtBot/bitcoin/builds/31864368

ACKs for top commit:
  achow101:
    ACK fa2251df5e3471e0d19d5d5c7780d34ee1498cee

Tree-SHA512: 9450d216d9989d6c44028ae4b9818790cfb00796e0de22331422f775f74d697bb14ebae0e88dca20c6b641363780da384fe94c708e20fce9cfde929fb343b12f

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