Element-R: support for starting a SAS verification (#3528)

* integ tests: factor out some utility methods

* Add `VerificationRequest.startVerification` to replace `beginKeyVerification`

The rust SDK ties together creating the verifier and sending the
`m.key.verification.start` message, so we need to combine
`.beginKeyVerification` and `.verify`.

* add some unit tests

spec/unit/rust-crypto/verification.spec.ts

@@ -0,0 +1,50 @@
+/*
+Copyright 2023 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import * as RustSdkCryptoJs from "@matrix-org/matrix-sdk-crypto-js";
+import { Mocked } from "jest-mock";
+
+import { RustVerificationRequest } from "../../../src/rust-crypto/verification";
+import { OutgoingRequestProcessor } from "../../../src/rust-crypto/OutgoingRequestProcessor";
+
+describe("VerificationRequest", () => {
+    describe("startVerification", () => {
+        let mockedInner: Mocked<RustSdkCryptoJs.VerificationRequest>;
+        let mockedOutgoingRequestProcessor: Mocked<OutgoingRequestProcessor>;
+        let request: RustVerificationRequest;
+
+        beforeEach(() => {
+            mockedInner = {
+                registerChangesCallback: jest.fn(),
+                startSas: jest.fn(),
+            } as unknown as Mocked<RustSdkCryptoJs.VerificationRequest>;
+            mockedOutgoingRequestProcessor = {} as Mocked<OutgoingRequestProcessor>;
+            request = new RustVerificationRequest(mockedInner, mockedOutgoingRequestProcessor);
+        });
+
+        it("does not permit methods other than SAS", async () => {
+            await expect(request.startVerification("m.reciprocate.v1")).rejects.toThrow(
+                "Unsupported verification method",
+            );
+        });
+
+        it("raises an error if starting verification does not produce a verifier", async () => {
+            await expect(request.startVerification("m.sas.v1")).rejects.toThrow(
+                "Still no verifier after startSas() call",
+            );
+        });
+    });
+});