test: support run, debug commands in basic NixOS tests

Currently, this only affects the basic NixOS test `clightning-replication`.

test/lib/make-test-vm.nix

@@ -27,4 +27,9 @@ let
     inherit (test) meta passthru;
   } // test;
 in
-  runTest
+  runTest // {
+    # A VM runner for interactive use
+    run = pkgs.writers.writeBashBin "run-vm" ''
+      . ${./run-vm.sh} ${runTest.driver} "$@"
+    '';
+  }