Use absolute UNIX socket paths for test Gitaly processes in CI

Instead of using relative paths, we use absolute paths for the Gitaly
processes run in CI. This attempts to rule out relative path issues are
causing random spec failures in
https://gitlab.com/gitlab-org/gitlab/-/issues/297359.

spec/support/helpers/test_env.rb

@@ -192,7 +192,17 @@ module TestEnv
   end
 
   def gitaly_dir
-    File.dirname(gitaly_socket_path)
+    socket_path = gitaly_socket_path
+    socket_path = File.expand_path(gitaly_socket_path) if expand_path?
+
+    File.dirname(socket_path)
+  end
+
+  # Linux fails with "bind: invalid argument" if a UNIX socket path exceeds 108 characters:
+  # https://github.com/golang/go/issues/6895. We use absolute paths in CI to ensure
+  # that changes in the current working directory don't affect GRPC reconnections.
+  def expand_path?
+    !!ENV['CI']
   end
 
   def start_gitaly(gitaly_dir)