Add BTC testnet4 servers.json file so that peering works on BTC test4

Currently only contains 1 entry, blackie.c3-soft.com (which is not yet
up), but this 1 entry will serve as the seed to find all the other
servers, as they may appear.

Note that Core only just recently merged testnet4 chain to master so
there is no real rush to get other servers up right now, but we will be
ready for it on the Fulcrum side as more servers appear.

Core added testnet4 here: bitcoin/bitcoin#29775

resources.qrc

@@ -7,6 +7,7 @@
         <file>resources/bch/servers_chipnet.json</file>
         <file>resources/btc/servers.json</file>
         <file>resources/btc/servers_testnet.json</file>
+        <file>resources/btc/servers_testnet4.json</file>
         <file>resources/ltc/servers.json</file>
         <file>resources/ltc/servers_testnet.json</file>
     </qresource>

resources/btc/servers_testnet4.json

@@ -0,0 +1,8 @@
+{
+    "blackie.c3-soft.com": {
+        "pruning": "-",
+        "s": "57010",
+        "t": "57009",
+        "version": "1.4"
+    }
+}

src/PeerMgr.cpp

@@ -96,7 +96,7 @@ void PeerMgr::startup()
     else if (net == BTC::Net::TestNet)
         parseServersDotJson(pathPrefix + "servers_testnet.json");
     else if (net == BTC::Net::TestNet4)
-        parseServersDotJson(pathPrefix + "servers_testnet4.json"); // BCH only -- will implicitly throw if somehow the coin is BTC (should never happen)
+        parseServersDotJson(pathPrefix + "servers_testnet4.json"); // BCH & BTC only -- will implicitly throw if somehow the coin is LTC (should never happen)
     else if (net == BTC::Net::ScaleNet)
         parseServersDotJson(pathPrefix + "servers_scalenet.json"); // BCH only -- will implicitly throw if somehow the coin is BTC (should never happen)
     else if (net == BTC::Net::ChipNet)