Merge #19455: rpc generate: print useful help and error message

f0aa8aeea5 test: add rpc_generate functional test (Jon Atack)
92d94ffb8d rpc: print useful help and error message for generate (Jon Atack)
8d32d2011d test: consider generate covered in _get_uncovered_rpc_commands() (Jon Atack)

Pull request description:

  This was a requested follow-up to #19133 and #17700 to alleviate confusion and head-scratching by people following tutorials that use `generate`. See https://github.com/bitcoin/bitcoin/pull/19455#issuecomment-668172916 below, https://github.com/bitcoin/bitcoin/pull/19133#issuecomment-636860943 and https://github.com/bitcoin/bitcoin/pull/17700#issuecomment-566159096.

  before
  ```
  $ bitcoin-cli help generate
  help: unknown command: generate

  $ bitcoin-cli generate
  error code: -32601
  error message:
  Method not found
  ```

  after
  ```
  $ bitcoin-cli help generate
  generate ( nblocks maxtries ) has been replaced by the -generate cli option. Refer to -help for more information.

  $ bitcoin-cli generate
  error code: -32601
  error message:
  generate ( nblocks maxtries ) has been replaced by the -generate cli option. Refer to -help for more information.
  ```

  In the general help it remains hidden, as requested by laanwj.
  ```
  $ bitcoin-cli help

  == Generating ==
  generateblock "output" ["rawtx/txid",...]
  generatetoaddress nblocks "address" ( maxtries )
  generatetodescriptor num_blocks "descriptor" ( maxtries )
  ```

ACKs for top commit:
  adamjonas:
    utACK f0aa8aeea5
  pinheadmz:
    ACK f0aa8aeea5

Tree-SHA512: d083652589ad3e8228c733455245001db22397559c3946e7e573cf9bd01c46e9e88b72d934728ec7f4361436ae4c74adb8f579670b09f479011924357e729af5

test/functional/test_runner.py

@@ -194,6 +194,7 @@ BASE_SCRIPTS = [
     'p2p_eviction.py',
     'rpc_signmessage.py',
     'rpc_generateblock.py',
+    'rpc_generate.py',
     'wallet_balance.py',
     'feature_nulldummy.py',
     'mempool_accept.py',
@@ -713,14 +714,16 @@ class RPCCoverage():
         Return a set of currently untested RPC commands.
 
         """
-        # This is shared from `test/functional/test-framework/coverage.py`
+        # This is shared from `test/functional/test_framework/coverage.py`
         reference_filename = 'rpc_interface.txt'
         coverage_file_prefix = 'coverage.'
 
         coverage_ref_filename = os.path.join(self.dir, reference_filename)
         coverage_filenames = set()
         all_cmds = set()
-        covered_cmds = set()
+        # Consider RPC generate covered, because it is overloaded in
+        # test_framework/test_node.py and not seen by the coverage check.
+        covered_cmds = set({'generate'})
 
         if not os.path.isfile(coverage_ref_filename):
             raise RuntimeError("No coverage reference found")