Adds a simple test for Signup component

app/client/src/components/Signup/Signup.test.js

+import React from 'react';
+import { render, getByText } from '@testing-library/react';
+import Signup from './Signup';
+
+describe('<Signup /> component', () => {
+    it('renders a Signup contents', () => {
+        const { container, getByText } = render(<Signup />);
+        const main_container = container.querySelector('.login-todo-main')
+        expect(main_container).toBeVisible();
+
+        expect(main_container).toHaveTextContent('Your name');
+        expect(main_container).toHaveTextContent('Your email');
+        expect(main_container).toHaveTextContent('Your password');
+
+        const button_create = getByText('Create User');
+        expect(button_create).toBeVisible();
+
+        const button_singin = getByText('Sign-in');
+        expect(button_singin).toBeVisible();
+    });
+});