Fix failing clone_push_pull snippet tests

Change add_to_file_content method
To use send_keys so that it adds text
in addition to the existing one

See https://gitlab.com/gitlab-org/gitlab/-/issues/335318

qa/qa/page/dashboard/snippet/edit.rb

@@ -23,7 +23,8 @@ module QA
           end
 
           def add_to_file_content(content)
-            text_area.set content
+            text_area.click
+            text_area.send_keys(:home, content) # starts in the beginning of the line
             text_area.has_text?(content) # wait for changes to take effect
           end