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

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

Need to be run with --coverage

Backport of Core PR15943

Test Plan:

test_runner.py --coverage --extended

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

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