added tests for button component

src/components/Button/Button.js

 
   return (
     <button
+      data-testid="btn"
       ref={buttonRef}
       className={styles()}
       onClick={handleClick}

src/components/Button/Button.test.js

+import Button from './Button'
+import ReactDOM from 'react-dom'
+import React from 'react'
+import { getByTestId, render, screen } from '@testing-library/react';
+
+describe("Button tests", () =>{
+    it("render without crashing", () =>
+    {
+        const div = document.createElement('div');
+        ReactDOM.render(<Button></Button>, div);
+    })
+
+    it("render button with base class",() =>
+    {
+        render(<Button></Button>);
+        expect(screen.getByTestId('btn')).toHaveClass("c-btn");
+    })
+    
+    it("render button with size 5",() =>
+    {
+        render(<Button size={5}></Button>);
+        expect(screen.getByTestId('btn')).toHaveClass("c-btn--5");
+    })
+})