jumper.vim - Easiest way to toggle files.
Add to your .vimrc:
NeoBundle 'minamorl/jumper.vim'
let g:jumper_popup_style = "vsplit"
let g:jumper_config = {}
let g:jumper_config.python = "../tests/test_{}"
nnoremap <silent> <leader>t :call jumper#jump()<cr>
'{}'
is a holderplace that replesents current filename.
For example, if you are in 'example/example.py', press "<leader>t", then jumper.vim looks up filetype and opens "test/test_example.py"
by defined rules.