Merge #15943: tests: Fail if RPC has been added without tests

Summary:
fad0ce59e9 tests: Fail if RPC has been added without tests (MarcoFalke)

Pull request description:

Need to be run with --coverage

ACKs for commit fad0ce:

ryanofsky:
  utACK fad0ce59e9154f9b7e61907a71c740a942c60282. New comment in travis.yml is the only change since last review.

Tree-SHA512: b53632dfe9865ec06991bfcba2fd67238bebbb866b355f09624eaf233257b2bca902caac6c24abb358b2f4c1c43f28ca75e30982765911e1a117102df65276d9

Backport of Core PR15943

Test Plan:

test_runner.py --coverage --extended

Verify All RPC commands covered. is printed to the console.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5743