Use FileUtils.mkdir_p for ujs log directory

[zzak]

Noticed this when debugging the ujs tests for buildkite locally.

This change ensures that this task can be rerun.

For example:

$ mkdir log && ruby -e 'Dir.mkdir("log")'
-e:1:in `mkdir': File exists @ dir_s_mkdir - log (Errno::EEXIST)
        from -e:1:in `<main>'