Prevent rust-crypto setting from leaking to other tests (#10464)

Follow-up to #9759.

Because the MatrixClientPeg pushes the use_rust_crypto setting back into the SettingsStore, the setting was leaking out to other tests despite getValue's mock being restored.

The solution is to mock out setValue as well.

test/MatrixClientPeg-test.ts

@@ -20,6 +20,7 @@ import fetchMockJest from "fetch-mock-jest";
 import { advanceDateAndTime, stubClient } from "./test-utils";
 import { IMatrixClientPeg, MatrixClientPeg as peg } from "../src/MatrixClientPeg";
 import SettingsStore from "../src/settings/SettingsStore";
+import { SettingLevel } from "../src/settings/SettingLevel";
 
 jest.useFakeTimers();
 
@@ -121,12 +122,17 @@ describe("MatrixClientPeg", () => {
                 },
             );
 
+            const mockSetValue = jest.spyOn(SettingsStore, "setValue").mockResolvedValue(undefined);
+
             const mockInitCrypto = jest.spyOn(testPeg.get(), "initCrypto").mockResolvedValue(undefined);
             const mockInitRustCrypto = jest.spyOn(testPeg.get(), "initRustCrypto").mockResolvedValue(undefined);
 
             await testPeg.start();
             expect(mockInitCrypto).not.toHaveBeenCalled();
             expect(mockInitRustCrypto).toHaveBeenCalledTimes(1);
+
+            // we should have stashed the setting in the settings store
+            expect(mockSetValue).toHaveBeenCalledWith("feature_rust_crypto", null, SettingLevel.DEVICE, true);
         });
     });
 });