Switch crypto.spec.ts away from TestClient and matrix-mock-request. (#3142)

I became sufficiently annoyed with matrix-mock-request that I decided to replace it with fetch-mock, which is what we use in matrix-react-sdk and is generally more powerful, easier to use, and actually maintained.

Unfortunately, we have a TestClient utility which is widely used and quite tightly integrated with matrix-mock-request. It wasn't going to be possible to change TestClient without changing all the tests that use it.

I also don't find TestClient particularly easy to use - it does a lot of stuff which I'm not convinced ought to be done for every single test.

So... I've introduced a couple of new classes (SyncResponder, E2EKeyReceiver) which do some of the useful bits of TestClient, but in a more granular way, and have switched crypto.spec.ts over so that rather than instantiating a TestClient for each test, it creates a MatrixClient directly and intercepts the endpoints necessary.

spec/integ/olm-encryption-spec.ts

@@ -29,13 +29,13 @@ limitations under the License.
 import "../olm-loader";
 
 import type { Session } from "@matrix-org/olm";
+import type { IDeviceKeys, IOneTimeKey } from "../../src/@types/crypto";
 import { logger } from "../../src/logger";
 import * as testUtils from "../test-utils/test-utils";
 import { TestClient } from "../TestClient";
 import { CRYPTO_ENABLED, IClaimKeysRequest, IQueryKeysRequest, IUploadKeysRequest } from "../../src/client";
 import { ClientEvent, IContent, ISendEventResponse, MatrixClient, MatrixEvent } from "../../src/matrix";
 import { DeviceInfo } from "../../src/crypto/deviceinfo";
-import { IDeviceKeys, IOneTimeKey } from "../../src/crypto/dehydration";
 
 let aliTestClient: TestClient;
 const roomId = "!room:localhost";