tests/regtest: disable incompatible validatepegin for liquidd

Otherwise liquidd startup fails. This bug was not covered by our tests,
because we're not combining `regtest` with `secure-node`.
But nixbitcoin.org does, which should suffice for now.

modules/liquid.nix

@@ -234,6 +234,12 @@ in {
   inherit options;
 
   config = mkIf cfg.enable {
+    assertions = [
+      { assertion = bitcoind.regtest -> cfg.validatepegin != true;
+        message = "liquidd: `validatepegin` is incompatible with regtest.";
+      }
+    ];
+
     services.bitcoind.enable = true;
 
     environment.systemPackages = [

test/tests.nix

@@ -248,6 +248,9 @@ let
       services.lightning-pool.extraConfig = ''
         auctionserver=localhost
       '';
+
+      # `validatepegin` is incompatible with regtest
+      services.liquidd.validatepegin = mkForce false;
     };
 
     ## Examples / debug helper