go: test: factor out mosaicPrepare
go: test: factor out mosaicPrepare

It is to be used from other tests, too.

Also, rename:
  prepare -> dirPrepare
  cleanup -> dirCleanup

Signed-off-by: Kir Kolyshkin <c0159612b0ef32139322233232df1a0d189c4b9b@openvz.org>

kolyshkin/mosaic,kolyshkin/mosaic,kolyshkin/mosaic