miner: have waitNext return after 20 min on testnet

On testnet we need to create a min diff template after 20 min.

src/node/interfaces.cpp

@@ -958,6 +958,7 @@ public:
         auto now{NodeClock::now()};
         const auto deadline = now + options.timeout;
         const MillisecondsDouble tick{1000};
+        const bool allow_min_difficulty{chainman().GetParams().GetConsensus().fPowAllowMinDifficultyBlocks};
 
         do {
             bool tip_changed{false};
@@ -982,6 +983,14 @@ public:
             // Must release m_tip_block_mutex before locking cs_main, to avoid deadlocks.
             LOCK(::cs_main);
 
+            // On test networks return a minimum difficulty block after 20 minutes
+            if (!tip_changed && allow_min_difficulty) {
+                const NodeClock::time_point tip_time{std::chrono::seconds{chainman().ActiveChain().Tip()->GetBlockTime()}};
+                if (now > tip_time + 20min) {
+                    tip_changed = true;
+                }
+            }
+
             /**
              * We determine if fees increased compared to the previous template by generating
              * a fresh template. There may be more efficient ways to determine how much