Merge #18263: rpc: change setmocktime check to use IsMockableChain

2455aa5 [rpc] changed MineBlocksOnDemand to IsMockableChain (Gloria Zhao)

Pull request description:

  Change: Update the if statement in `setmocktime` to use `IsMockableChain` chainparams function (aka `m_is_mockable_chain`) instead of `MineBlocksOnDemand`

  Rationale: It's a more appropriate check for whether or not chain is in RegTest, as [discussed](bitcoin/bitcoin#18037 (comment)) in #18037

ACKs for top commit:
  MarcoFalke:
    ACK 2455aa5 🙇
  jonatack:
    ACK 2455aa5

Tree-SHA512: 1d8c8b7ff0b3c1bcbf5755194969b6664fe05a35003375ad08d18e34bcefd2df4f64d0e60078a10bbef3c8f469a9b9d07db467089b55c14cf532304bc965bffc

Author: MarcoFalke

src/rpc/misc.cpp

@@ -351,8 +351,9 @@ static UniValue setmocktime(const JSONRPCRequest& request)
                 RPCExamples{""},
             }.Check(request);
 
-    if (!Params().MineBlocksOnDemand())
-        throw std::runtime_error("setmocktime for regression testing (-regtest mode) only");
+    if (!Params().IsMockableChain()) {
+        throw std::runtime_error("setmocktime is for regression testing (-regtest mode) only");
+    }
 
     // For now, don't change mocktime if we're in the middle of validation, as
     // this could have an effect on mempool time-based eviction, as well as