Merge #14658: qa: Add test to ensure node can generate all rpc help texts at runtime

Summary:
bbbbb3f8850907d413db4715c10ef6df055234f6 qa: Add test to ensure node can generate all help texts at runtime (MarcoFalke)

Pull request description:

This might increase coverage, but more importantly this checks that the node doesn't crash when generating the help. (Right now the help is a static string, but in the future it might be generated at runtime)

Tree-SHA512: 0226e7c65f8a1a6fdc96c07dcf491d90559bc2355c92e9da9b1f174b09733fc349269e71da6d792f954de563a1e57c848471813eabae1a40b849a0d989520a0d

Backport of Core PR14658

Test Plan: test_runner.py rpc_help

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6709