Check for existence of gitaly-ruby Gemfile

This will make it clearer why a gitaly-ruby `bundle check`
failed. Currently there is an implicit assumption that jobs in the test
stage will run within 24 hours of the `setup-test-env` step
running. However, if a test runs after that time frame, the CI artifacts
will have been destroyed, and the `tmp/tests` directory won't contain
the right data.

This happened in https://gitlab.com/gitlab-org/gitlab/-/jobs/371020233.
The gitaly-ruby `bundle check` quietly failed because
`tmp/tests/gitaly-ruby/Gemfile` didn't exist. At first, we thought
`bundle check` failed because of a stale cache or a missing Gem.

scripts/gitaly_test.rb

@@ -56,6 +56,14 @@ module GitalyTest
   end
 
   def check_gitaly_config!
+    puts "Checking gitaly-ruby Gemfile..."
+
+    unless File.exist?(gemfile)
+      message = "#{gemfile} does not exist."
+      message += "\n\nThis might have happened if the CI artifacts for this build were destroyed." if ENV['CI']
+      abort message
+    end
+
     puts 'Checking gitaly-ruby bundle...'
     abort 'bundle check failed' unless system(env, 'bundle', 'check', chdir: File.dirname(gemfile))
   end