…ut value only once per UTXO)

82dee87 test: test decodepsbt fee calculation (count input value only once per UTXO) (Sebastian Falbesoner)

Pull request description:

  Fixes bitcoin#19523, adding a simple test to `rpc_psbt.py` that checks that the decodepsbt fee matches the one given by the wallet (`walletcreatefundedpsbt`). This is in particular important for PSBTs with segwit inputs that have both a witness- and a non-witness-UTXO type set.

  Example test run after reverting commit 7512278 ("Increment input value sum only once per UTXO in decodepsbt"):

  ```
  $ test/functional/rpc_psbt.py
  2020-07-26T11:31:44.862000Z TestFramework (INFO): Initializing test directory /tmp/bitcoin_func_test__sutcd4y
  20.00007580
  2020-07-26T11:31:47.073000Z TestFramework (ERROR): Assertion failed
  Traceback (most recent call last):
    File "/home/honeybadger/buidl/bitcoin_thestack/test/functional/test_framework/test_framework.py", line 118, in main
      self.run_test()
    File "test/functional/rpc_psbt.py", line 166, in run_test
      assert_equal(decoded['fee'], created_psbt['fee'])
    File "/home/honeybadger/buidl/bitcoin_thestack/test/functional/test_framework/util.py", line 49, in assert_equal
      raise AssertionError("not(%s)" % " == ".join(str(arg) for arg in (thing1, thing2) + args))
  AssertionError: not(20.00007580 == 0.00007580)
  2020-07-26T11:31:47.125000Z TestFramework (INFO): Stopping nodes
  ......
  ```

ACKs for top commit:
  achow101:
    ACK 82dee87

Tree-SHA512: 296b8a701f851d482ef6200c6cbf0cf0257a79a828ac6dbc39b05d8c2d839c6fdb9d3f5a084015295cfa3eac7c11faa2f2d52e619c11627b04c75150eead8330