I don't use vim-rubytest plugin but looking at the documentation it seems to me that by default vim-rubytest will put the contents into the quickfix list. To open the quickfix window issue the following command:
:copen
You can navigate the quickfix list via :cnext
and :cprevious
.
I belive the quickfix approach to be the preferred way, but to answer you question you can redirect the output into a register and then paste into a new buffer.
:redir @"
Then execute <leader>T
. watch it all go by. Then end the redirection and create a new buffer with the contents inside.
:redir END
:new|pu|0d_
For more help see
:h quickfix
:h :cnext
:h :redir
:h :new
:h :put
:h :delete