Ensure we save QA screenshots to an absolute path

This allows to open the file address printed in the terminal, otherwise
the address started with `tmp` which was relatove to the `qa/` dir.
Signed-off-by: Rémy Coutable <remy@rymai.me>

qa/qa/runtime/browser.rb

@@ -84,7 +84,7 @@ module QA
           config.javascript_driver = :chrome
           config.default_max_wait_time = 10
           # https://github.com/mattheworiordan/capybara-screenshot/issues/164
-          config.save_path = 'tmp'
+          config.save_path = File.expand_path('../../tmp', __dir__)
         end
       end