TT#68016 selenium: take screenshot when alert check fails

FirefoxExtensions.pm:
- take a screenshot when a alert check fails. can store multiple failed alert
 checks if needed

Change-Id: I6f14974960bca9b6221af7cf3619bbe808bb207b
(cherry picked from commit 339dcbbb86)

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

@@ -113,6 +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)) . '.png');
         return 'Element not found';
     };
 }