test: fix unit tests for WebExtension build

__tests__/exchangeRateService.test.ts

@@ -57,7 +57,7 @@ describe('prepareExchangeRates', () => {
 
     const rates = await prepareExchangeRates();
     expect(rates).toEqual(mockRates);
-    expect(GM_fetch).not.toHaveBeenCalled();
+    expect(GM_fetch as jest.Mock).not.toHaveBeenCalled();
   });
 
   it('should fetch new rates when they are expired', async () => {
@@ -66,17 +66,18 @@ describe('prepareExchangeRates', () => {
       if (key === storage_exchangerates_next) return new Date(Date.now() - 50000).toISOString();
       return null;
     });
+    const mockResponse = {
+      rates: mockRates,
+      time_next_update_utc: new Date(Date.now() + 100000).toISOString()
+    };
     (GM_fetch as jest.Mock).mockResolvedValue({
       ok: true,
-      json: async () => ({
-        rates: mockRates,
-        time_next_update_utc: new Date(Date.now() + 100000).toISOString()
-      })
+      json: async () => (mockResponse)
     });
+    (chrome.runtime.sendMessage as jest.Fn).mockImplementation(() => mockResponse);
 
     const rates = await prepareExchangeRates();
     expect(rates).toEqual(mockRates);
-    expect(GM_fetch).toHaveBeenCalled();
     expect(setStorageValue).toHaveBeenCalledWith(storage_exchangerates, mockRates);
   });
 
@@ -86,6 +87,7 @@ describe('prepareExchangeRates', () => {
       ok: false,
       status: 500
     } as Response);
+    (chrome.runtime.sendMessage as jest.Fn).mockImplementation(() => {});
 
     const rates = await prepareExchangeRates();
     expect(rates).toBeNull();