Allow any formatter when running in parallel (-j)

[eregon]

Currently, the MultiFormatter always uses the SpinnerFormatter but this is not always appropriate (e.g.: CI).

[eregon]

CRuby is working around this with https://github.com/ruby/ruby/blob/c0256d1d5d397991e4f2dec6359339f406aa3dd8/spec/default.mspec#L92-L161