[qa] Fix assertions in fundrawtransaction
[qa] Fix assertions in fundrawtransaction

Summary:
  As per title

Test Plan:
  ./test/functional/test_runner.py fundrawtransaction

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1645

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