Fix xdebug and pcov on PHP7.4 and restructre code

__tests__/pecl.test.ts

@@ -0,0 +1,15 @@
+import * as pecl from '../src/pecl';
+
+let valid_extensions = ['xdebug', 'pcov'];
+jest.mock('../src/pecl', () => ({
+  checkPECLExtension: jest.fn().mockImplementation(extension => {
+    return valid_extensions.indexOf(extension) !== -1;
+  })
+}));
+
+describe('pecl tests', () => {
+  it('checking checkPECLExtension', async () => {
+    expect(await pecl.checkPECLExtension('extensionDoesNotExist')).toBe(false);
+    expect(await pecl.checkPECLExtension('xdebug')).toBe(true);
+  });
+});