Add preliminary testing option for block v11 height on testnet

This allows for configuration of the testnet block version 11 transition
height for early testing of hard fork changes. Specify -v11height=X from
the command line or add the directive to the configuration file to start
a node that will switch to the block version 11 protocol after the chain
grows to X number of blocks.

src/init.cpp

@@ -533,6 +533,8 @@ bool AppInit2(ThreadHandlerPtr threads)
         }
     }
 
+    LogPrintf("Block version 11 hard fork configured for %d", GetV11Threshold());
+
     if (NN::Quorum::Active())
     {
         LogPrintf("INFO: Native C++ neural network is active.");

src/main.h

@@ -123,9 +123,16 @@ inline bool IsV10Enabled(int nHeight)
 inline int32_t GetV11Threshold()
 {
     // Returns "never" before planned intro of bv11.
-    return fTestNet
-            ? std::numeric_limits<int32_t>::max()
-            : std::numeric_limits<int32_t>::max();
+    try {
+        return fTestNet
+                // Temporary: configure testnet v11 height via parameter before
+                // releasing v11 to regular testnet:
+                //
+                ? std::stoi(GetArg("-v11height", ""))
+                : std::numeric_limits<int32_t>::max();
+    } catch (...) {
+        return std::numeric_limits<int32_t>::max();
+    }
 }
 
 inline bool IsV11Enabled(int nHeight)