tests: add case normalizer to cover sandbox.

src/__tests__/lib/normalizer.test.ts

@@ -124,5 +124,43 @@ describe("lib/normalizer: ", () => {
       // @ts-ignore
       cb(null, mailData);
     });
+
+    it("checks if testing client is switched.", () => {
+      expect.assertions(3);
+
+      const mockResponse = {
+        statusCode: 200,
+      };
+      const mockClient = {
+        testing: {
+          send: jest.fn(() => Promise.resolve(mockResponse)),
+        },
+      };
+
+      const callback = (error: Error, data: SendError) => {
+        expect(error).toBeNull();
+        expect(data).toEqual(mockResponse);
+      };
+      const mailData = {
+        sandbox: true,
+        text: "mock-text",
+        to: {
+          address: "mock@mail.com",
+          name: "mock-name",
+        },
+        from: {
+          address: "mock@mail.com",
+          name: "mock-name",
+        },
+        subject: "mock-subject",
+      };
+
+      // @ts-ignore
+      const cb = normalizeCallback(mockClient, callback);
+
+      // @ts-ignore
+      cb(null, mailData);
+      expect(mockClient.testing.send).toBeCalledTimes(1);
+    });
   });
 });