[nix] add base integration test

run it using 'nix flake check -L'

test/test.nix

@@ -0,0 +1,42 @@
+# ./tests/hello-world-server.nix
+(import ./lib.nix) {
+  name = "gokill-base-test";
+  nodes = {
+    # `self` here is set by using specialArgs in `lib.nix`
+    node1 = { self, pkgs, ... }: {
+      imports = [ self.nixosModules.gokill ];
+
+      services.gokill = {
+        enable = true;
+        triggers = [
+          {
+            type = "Timeout";
+            name = "custom timeout";
+            options = {
+              duration =  10;
+            };
+            actions = [
+              {
+                  type = "Command";
+                  options = {
+                      command = "echo hello world";
+                  };
+                  stage = 2;
+              }
+            ];
+          }
+        ];
+      };
+    };
+  };
+
+  testScript = ''
+    import time
+    start_all() # wait for our service to start
+    node1.wait_for_unit("gokill")
+    time.sleep(11)
+    output = node1.succeed("journalctl -u gokill.service | tail -n 2 | head -n 1")
+    # Check if our webserver returns the expected result
+    assert "hellow world" in output
+  '';
+}