test: better naming for e2e screenshots

e2e/app.e2e-spec.ts

@@ -7,7 +7,7 @@ describe('Copay', () => {
   });
 
   it('Should display the logo on the landing view', async () => {
-    takeScreenshot('landing.png');
+    takeScreenshot('landing');
     const present = await element(by.css('#logo')).isPresent();
     expect(present).toEqual(true);
   });

e2e/screenshots.ts

@@ -15,7 +15,7 @@ export async function takeScreenshot(name: string) {
   const config = await browser.getProcessedConfig();
   const deviceName = config['capabilities']['name'];
   const pngData = await browser.takeScreenshot();
-  const path = join(dir, `${name}_${deviceName}`);
+  const path = join(dir, `${name}_${deviceName}.png`);
   writeFile(path, pngData, { encoding: 'base64' }, err => {
     err ? console.error(err) : console.log(`File written: ${path}`);
   });