install: add unit tests

Author: yasminvalim

src/cmdline/install.rs

@@ -584,4 +584,24 @@ dest-device: u
         .expand_config_files()
         .unwrap_err();
     }
+
+    /// Test that firstboot-args is manually added to args list when defined
+    #[test]
+    fn test_firstboot_args_manually_added() {
+        let mut f = NamedTempFile::new().unwrap();
+        f.as_file_mut()
+            .write_all(b"dest-device: /dev/sda")
+            .unwrap();
+
+        let config = InstallConfig::from_args(&[
+            "--config-file",
+            f.path().to_str().unwrap(),
+            "--firstboot-args",
+            "ip=dhcp",
+        ])
+        .unwrap();
+
+        // Test expand_config_files to verify manual addition
+        let expanded = config.expand_config_files().unwrap();
+    }
 }