TT#68016 selenium: add proper file type to alert screenshots

FirefoxExtensions.pm:
- add .png file type to screenshots so you can view it directly
in your browser instead of having to download the file

imagine needing 3 commits to get a line of code working properly (:

Change-Id: I864fb06ae7adcd686a9af1fe06ce70d3c90fbba5

t/lib/Selenium/Remote/Driver/FirefoxExtensions.pm

@@ -113,7 +113,7 @@ sub get_text_safe {
         my $element = $self->find_element($path, $type);
         return $element->get_text();
     } catch {
-        $self->capture_screenshot('/results/alert' . int(rand(1000)));
+        $self->capture_screenshot('/results/alert' . int(rand(1000)) . '.png');
         return 'Element not found';
     };
 }