6af6d9b23d test: Add tests for RPC help (João Barbosa)
Pull request description:
At the moment the new test checks for:
- invalid usages
- expected output for unknown command
- current RPC command titles (derived from command categories) — this prevents adding wrong RPC categories and new categories must be added to the test
Tree-SHA512: f987535d001b1cd300656588602b1634099ea68a1dd2282180c30fa56caf7f990be9e2dc86c7431dfcf7fd686d0299a8d4935df178a2c9f0fb6fbebcba748eb5
