Improve update UI error details

Sources/Update/UpdateTestSupport.swift

@@ -0,0 +1,48 @@
+#if DEBUG
+import Foundation
+import Sparkle
+
+enum UpdateTestSupport {
+    static func applyIfNeeded(to viewModel: UpdateViewModel) {
+        let env = ProcessInfo.processInfo.environment
+        guard env["CMUX_UI_TEST_MODE"] == "1" else { return }
+        guard let state = env["CMUX_UI_TEST_UPDATE_STATE"] else { return }
+
+        DispatchQueue.main.async {
+            switch state {
+            case "available":
+                let version = env["CMUX_UI_TEST_UPDATE_VERSION"] ?? "9.9.9"
+                transition(to: .updateAvailable(.init(
+                    appcastItem: makeAppcastItem(displayVersion: version) ?? SUAppcastItem.empty(),
+                    reply: { _ in }
+                )), on: viewModel)
+            case "notFound":
+                transition(to: .notFound(.init(acknowledgement: {})), on: viewModel)
+            default:
+                break
+            }
+        }
+    }
+
+    private static func transition(to state: UpdateState, on viewModel: UpdateViewModel) {
+        viewModel.state = .checking(.init(cancel: {}))
+        DispatchQueue.main.asyncAfter(deadline: .now() + 0.2) {
+            viewModel.state = state
+        }
+    }
+
+    private static func makeAppcastItem(displayVersion: String) -> SUAppcastItem? {
+        let enclosure: [String: Any] = [
+            "url": "https://example.com/cmux.zip",
+            "length": "1024",
+            "sparkle:version": displayVersion,
+            "sparkle:shortVersionString": displayVersion,
+        ]
+        let dict: [String: Any] = [
+            "title": "cmux \(displayVersion)",
+            "enclosure": enclosure,
+        ]
+        return SUAppcastItem(dictionary: dict)
+    }
+}
+#endif