Allow user-agent to be set in QA tests

Can be set via the `GITLAB_QA_USER_AGENT` environment variable.

The default Chrome user-agent will be used if the environment variable
is not set.

qa/qa/runtime/browser.rb

@@ -98,6 +98,10 @@ module QA
 
             # Disable /dev/shm use in CI. See https://gitlab.com/gitlab-org/gitlab/issues/4252
             options.add_argument("disable-dev-shm-usage") if QA::Runtime::Env.running_in_ci?
+
+            # Specify the user-agent to allow challenges to be bypassed
+            # See https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11938
+            options.add_argument("user-agent=#{QA::Runtime::Env.user_agent}") if QA::Runtime::Env.user_agent
           end
 
           # Use the same profile on QA runs if CHROME_REUSE_PROFILE is true.

qa/qa/runtime/env.rb

@@ -395,6 +395,10 @@ module QA
         ENV['DEPLOY_VERSION']
       end
 
+      def user_agent
+        ENV['GITLAB_QA_USER_AGENT']
+      end
+
       private
 
       def remote_grid_credentials