Provide a devtool for manually verifying other devices (#30094)

Also allows doing the same thing via a slash command.

test/unit-tests/SlashCommands-test.tsx

@@ -21,6 +21,7 @@ import { WidgetType } from "../../src/widgets/WidgetType";
 import { warnSelfDemote } from "../../src/components/views/right_panel/UserInfo";
 import dispatcher from "../../src/dispatcher/dispatcher";
 import { SettingLevel } from "../../src/settings/SettingLevel";
+import ErrorDialog from "../../src/components/views/dialogs/ErrorDialog";
 
 jest.mock("../../src/components/views/right_panel/UserInfo");
 
@@ -253,6 +254,20 @@ describe("SlashCommands", () => {
         });
     });
 
+    describe("/verify", () => {
+        it("should return usage if no args", () => {
+            const command = findCommand("verify")!;
+            expect(command.run(client, roomId, null, undefined).error).toBe(command.getUsage());
+        });
+
+        it("should show an error if device is not found", async () => {
+            const spy = jest.spyOn(Modal, "createDialog");
+            const command = findCommand("verify")!;
+            await command.run(client, roomId, null, "mydeviceid myfingerprint").promise;
+            expect(spy).toHaveBeenCalledWith(ErrorDialog, expect.objectContaining({ title: "Verification failed" }));
+        });
+    });
+
     describe("/addwidget", () => {
         it("should parse html iframe snippets", async () => {
             jest.spyOn(WidgetUtils, "canUserModifyWidgets").mockReturnValue(true);