Skip when unix socket path is too long

Eventually the path directly under "/tmp" is complained by `rm_r` in
spec/mspec/lib/mspec/helpers/fs.rb.

spec/ruby/library/socket/fixtures/classes.rb

@@ -37,7 +37,9 @@ module SocketSpecs
     # Check for too long unix socket path (max 104 bytes on macOS)
     # Note that Linux accepts not null-terminated paths but the man page advises against it.
     if path.bytesize > 104
-      path = "/tmp/unix_server_spec.socket"
+      # rm_r in spec/mspec/lib/mspec/helpers/fs.rb fails against
+      # "/tmp/unix_server_spec.socket"
+      skip "too long unix socket path: #{path}"
     end
     rm_socket(path)
     path