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.

src/client.ts

@@ -20,7 +20,7 @@ limitations under the License.
 
 import { Optional } from "matrix-events-sdk";
 
-import type { IMegolmSessionData } from "./@types/crypto";
+import type { IDeviceKeys, IMegolmSessionData, IOneTimeKey } from "./@types/crypto";
 import { ISyncStateData, SyncApi, SyncApiOptions, SyncState } from "./sync";
 import {
     EventStatus,
@@ -85,13 +85,7 @@ import { keyFromAuthData } from "./crypto/key_passphrase";
 import { User, UserEvent, UserEventHandlerMap } from "./models/user";
 import { getHttpUriForMxc } from "./content-repo";
 import { SearchResult } from "./models/search-result";
-import {
-    DEHYDRATION_ALGORITHM,
-    IDehydratedDevice,
-    IDehydratedDeviceKeyInfo,
-    IDeviceKeys,
-    IOneTimeKey,
-} from "./crypto/dehydration";
+import { DEHYDRATION_ALGORITHM, IDehydratedDevice, IDehydratedDeviceKeyInfo } from "./crypto/dehydration";
 import {
     IKeyBackupInfo,
     IKeyBackupPrepareOpts,