miner: have waitNext return after 20 min on testnet

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

src/test/util/setup_common.h

@@ -130,6 +130,12 @@ struct RegTestingSetup : public TestingSetup {
         : TestingSetup{ChainType::REGTEST} {}
 };
 
+/** Identical to TestingSetup, but chain set to testnet4 */
+struct Testnet4Setup : public TestingSetup {
+    Testnet4Setup()
+        : TestingSetup{ChainType::TESTNET4} {}
+};
+
 class CBlock;
 struct CMutableTransaction;
 class CScript;