test: Add a test case to products_createCategory_post.test tests file

Add a test case to the tests file of `products_createCategory_post` route handler for
testing the case that admins trying to add an existing category.

Fix a failure in the last test case in `products_createCategory_post.test` tests file.

__tests__/integration_testing/admin_routes_tests/products_createCategory_post.test.js

@@ -39,15 +39,27 @@ describe(`POST ${routePath}`, () => {
         const { productsCategories } = await SettingsModel.findOne({});
         expect(productsCategories.length).toBe(oldCategoriesLength + 1);
         expect(productsCategories.includes(newCategory)).toBe(true);
+    })
 
+    it("Should returns \"400\" error with \"The category already exist\" message", async () => {
+        const settings = await SettingsModel.findOne({}, ["productsCategories"]);
+        const existingCategory = settings.productsCategories[0];
+        const oldCategoriesLength = settings.productsCategories.length;
+        const response = await adminRequest(routePath + `?category=${existingCategory}`, "post");
+        expect(response.statusCode).toBe(400);
+        expect(response.body.message).toMatch("The category already exist");
+        expect(response.body.status).toMatch("Fail");
+        const { productsCategories } = await SettingsModel.findOne({});
+        expect(productsCategories.length).toBe(oldCategoriesLength);
+        expect(productsCategories.includes(existingCategory)).toBe(true);
     })
 
-    it("Should returns \"400\" error with \"You didn't provide a valid category name\" message", async () => {
+    it("Should returns \"400\" error with \"Ivalid Category Name\" message", async () => {
         const settings = await SettingsModel.findOne({}, ["productsCategories"]);
         const oldCategoriesLength = settings.productsCategories.length;
         const response = await adminRequest(routePath, "post");
         expect(response.statusCode).toBe(400);
-        expect(response.body.message).toMatch("You didn't provide a valid category name");
+        expect(response.body.message).toMatch("Ivalid Category Name");
         expect(response.body.status).toMatch("Fail");
         const { productsCategories } = await SettingsModel.findOne({});
         expect(productsCategories.length).toBe(oldCategoriesLength);