Enable the prepareToEncrypt test for Element-R (#3225)

This Just Works, so I don't know why I didn't enable the test sooner.

Author: richvdh

spec/integ/crypto.spec.ts

@@ -755,7 +755,7 @@ describe.each(Object.entries(CRYPTO_BACKENDS))("crypto (%s)", (backend: string,
         expect(event.getContent().body).toEqual("42");
     });
 
-    oldBackendOnly("prepareToEncrypt", async () => {
+    it("prepareToEncrypt", async () => {
         expectAliceKeyQuery({ device_keys: { "@alice:localhost": {} }, failures: {} });
         await startClientAndAwaitFirstSync();
         aliceClient.setGlobalErrorOnUnknownDevices(false);